Quarterly Results of Operations (Unaudited) (Results of Operations)(Details) - USD ($) $ in Thousands	3 Months Ended								6 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	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Total revenues	$ 47,874	$ 231,010	$ 140,925	$ 791,393	$ 662,173	$ 762,424	$ 601,620	$ 833,689	$ 932,318	$ 1,211,202	$ 2,859,906	$ 3,090,284
Total benefits and expenses	132,262	182,006	(588,114)	1,163,700	337,301	920,714	365,827	738,702	575,586	889,854	2,362,544	2,173,547
Income (loss) from operations before income taxes	(84,388)	49,004	729,039	(372,307)	324,872	(158,290)	235,793	94,987	356,732	321,348	497,362	916,737
Net income (loss)	$ (85,465)	$ 84,897	$ 723,984	$ (328,199)	$ 348,194	$ (136,506)	$ 214,381	$ 81,934	$ 395,785	$ 395,217	$ 508,003	$ 779,668