SCHEDULE II Condensed Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Total revenues	$ 124,535	$ 236,149	$ 121,478	$ 163,513	$ 188,735	$ 145,545	$ 87,810	$ 166,276	$ 645,675	$ 588,366	$ 123,050
Expenses
General and administrative expenses	5,064	8,202	6,347	6,743	6,878	6,937	4,994	6,999	26,356	25,808	23,434
Net income (loss) before equity in earnings of consolidated subsidiaries	(38,276)	21,018	(36,314)	8,747	50,310	31,297	(64,044)	29,646	(44,825)	47,209	(333,688)
Net income (loss)	$ (37,732)	$ 19,875	$ (35,469)	$ 8,374	$ 49,196	$ 30,011	$ (62,995)	$ 28,669	(44,952)	44,881	(326,425)
Parent Company
Revenues
Investment income									34,487	952	5,962
Total revenues									34,487	952	5,962
Expenses
General and administrative expenses									4,691	4,042	4,048
Net income (loss) before equity in earnings of consolidated subsidiaries									29,796	(3,090)	1,914
Equity in earnings of consolidated subsidiaries									(74,748)	47,971	(328,339)
Net income (loss)									$ (44,952)	$ 44,881	$ (326,425)