Quarterly Results of Operations (Unaudited) (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	$ 864,964	$ 272,685	$ (103,779)	$ (582,563)	$ 988,030	$ 826,028	$ 769,260	$ 1,331,262	$ 451,307	$ 3,914,580	$ 2,303,936
Total benefits and expenses	472,044	558,773	404,277	296,641	451,549	561,865	517,619	539,327	1,731,735	2,070,360	1,186,341
Income (loss) from operations before income taxes	392,920	(286,088)	(508,056)	(879,204)	536,481	264,163	251,641	791,935	(1,280,428)	1,844,220	1,117,595
Net income (loss)	$ 306,979	$ (227,512)	$ (168,770)	$ (900,024)	$ 432,255	$ 412,236	$ 202,546	$ 635,679	$ (989,327)	$ 1,682,716	$ (83,504)