Quarterly Results of Operations (Unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Total revenues	$ (181,460)	$ 719,985	$ (892,563)	$ 201,095	$ 219,952	$ 268,802	$ 279,135	$ 305,682	$ (152,943)	$ 1,073,571	$ 1,240,228
Total benefits and expenses	122,236	(149,250)	1,215,062	429,590	65,421	388,581	122,886	331,751	1,617,638	908,639	980,857
Income (loss) from operations before income taxes	(303,696)	869,235	(2,107,625)	(228,495)	154,531	(119,779)	156,249	(26,069)	(1,770,581)	164,932	259,371
Net income (loss)	$ (200,842)	$ 569,649	$ (1,316,230)	$ (142,665)	$ 167,431	$ (104,826)	$ 131,914	$ (21,302)	$ (1,090,088)	$ 173,217	$ 250,767