Summary of Significant Accounting Policies - Adoption of ASU 2014-09 (Details) - USD ($) $ / shares in Units, $ in Thousands |
3 Months Ended | 12 Months Ended | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2018 |
Sep. 30, 2018 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Consolidated Statements of Income: | |||||||||||
| Revenue | $ 93,201 | $ 73,168 | $ 73,844 | $ 60,007 | $ 39,957 | $ 51,977 | $ 32,293 | $ 30,407 | $ 300,220 | $ 154,634 | $ 66,109 |
| Taxes, other than income taxes | 265 | 167 | |||||||||
| Income taxes - deferred | 14,814 | 966 | (1,420) | ||||||||
| Net income | $ 62,680 | $ 50,762 | $ 52,503 | $ 43,791 | $ 24,620 | $ 33,002 | $ 20,350 | $ 19,259 | $ 209,736 | $ 97,231 | $ 42,275 |
| Net income per Sub-share Certificate (in dollars per share) | $ 8.06 | $ 6.52 | $ 6.73 | $ 5.60 | $ 3.14 | $ 4.20 | $ 2.58 | $ 2.43 | $ 26.93 | $ 12.38 | $ 5.29 |
| Assets: | |||||||||||
| Accrued receivables | $ 48,750 | $ 17,773 | $ 48,750 | $ 17,773 | |||||||
| Deferred tax liability | (114) | (114) | |||||||||
| Liabilities and Capital: | |||||||||||
| Unearned revenue | 13,369 | 8,364 | 13,369 | 8,364 | |||||||
| Other taxes payable | 0 | 0 | |||||||||
| Net proceeds from all sources | $ 245,769 | 105,902 | $ 245,769 | 105,902 | |||||||
| As reported | |||||||||||
| Consolidated Statements of Income: | |||||||||||
| Revenue | 132,329 | $ 59,911 | |||||||||
| Taxes, other than income taxes | 3,161 | 1,779 | |||||||||
| Income taxes - deferred | (3,365) | (4,194) | |||||||||
| Net income | $ 76,361 | $ 37,240 | |||||||||
| Net income per Sub-share Certificate (in dollars per share) | $ 9.72 | $ 4.66 | |||||||||
| Assets: | |||||||||||
| Accrued receivables | 18,205 | $ 18,205 | |||||||||
| Deferred tax asset | 6,992 | 6,992 | |||||||||
| Liabilities and Capital: | |||||||||||
| Unearned revenue | 41,375 | 41,375 | |||||||||
| Other taxes payable | 433 | 433 | |||||||||
| Net proceeds from all sources | 79,997 | 79,997 | |||||||||
| Accounting Standards Update 2014-09 | New Revenue Standard Adjustment | |||||||||||
| Consolidated Statements of Income: | |||||||||||
| Revenue | 22,305 | $ 6,198 | |||||||||
| Taxes, other than income taxes | (2,896) | (1,612) | |||||||||
| Income taxes - deferred | 4,331 | 2,774 | |||||||||
| Net income | $ 20,870 | $ 5,035 | |||||||||
| Net income per Sub-share Certificate (in dollars per share) | $ 2.66 | $ 0.63 | |||||||||
| Assets: | |||||||||||
| Accrued receivables | (432) | $ (432) | |||||||||
| Deferred tax asset | (7,106) | (7,106) | |||||||||
| Liabilities and Capital: | |||||||||||
| Unearned revenue | (33,011) | (33,011) | |||||||||
| Other taxes payable | (433) | (433) | |||||||||
| Net proceeds from all sources | $ 25,905 | $ 25,905 | |||||||||