Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ 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
Quarterly Financial Data [Abstract]
Total revenues	$ 988,030	$ 826,028	$ 769,260	$ 1,331,262	$ 314,778	$ 1,949,155	$ (726,666)	$ 766,669	$ 3,914,580	$ 2,303,936	$ (152,943)
Total benefits and expenses	451,549	561,865	517,619	539,327	367,400	597,242	(165,242)	386,941	2,070,360	1,186,341	1,617,638
Income (loss) from operations before income taxes	536,481	264,163	251,641	791,935	(52,622)	1,351,913	(561,424)	379,728	1,844,220	1,117,595	(1,770,581)
Net income (loss)	$ 432,255	$ 412,236	$ 202,546	$ 635,679	$ (884,205)	$ 938,926	$ (400,583)	$ 262,358	$ 1,682,716	$ (83,504)	$ (1,090,088)