Quarterly Results of Operations (Unaudited) (Results of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Total revenues	$ 220,388	$ 273,205	$ 233,359	$ 222,412	$ 209,146	$ 183,330	$ 248,288	$ 188,712	$ 949,364	$ 829,476	$ 687,965
Total benefits and expenses	185,977	234,568	219,237	150,798	200,479	166,760	224,981	179,469	790,580	771,689	516,515
Income (loss) from operations before income taxes and equity in earnings of operating joint venture	34,411	38,637	14,122	71,614	8,667	16,570	23,307	9,243	158,784	57,787	171,450
Net income (loss)	$ 37,403	$ 43,373	$ 48,435	$ 87,788	$ 42,383	$ 18,725	$ 36,226	$ 11,304	$ 216,999	$ 108,638	$ 327,793