Quarterly Results of Operations (Unaudited) (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
Quarterly Financial Data [Abstract]
Total revenues	$ 314,778	$ 1,949,155	$ (726,666)	$ 766,669	$ (181,460)	$ 719,985	$ (892,563)	$ 201,095	$ 2,303,936	$ (152,943)	$ 1,073,571
Total benefits and expenses	367,400	597,242	(165,242)	386,941	122,236	(149,250)	1,215,062	429,590	1,186,341	1,617,638	908,639
Income (loss) from operations before income taxes	(52,622)	1,351,913	(561,424)	379,728	(303,696)	869,235	(2,107,625)	(228,495)	1,117,595	(1,770,581)	164,932
Net income (loss)	$ (884,205)	$ 938,926	$ (400,583)	$ 262,358	$ (200,842)	$ 569,649	$ (1,316,230)	$ (142,665)	$ (83,504)	$ (1,090,088)	$ 173,217