Quarterly Results of Operations (Unaudited) (Results of Operations)(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	$ 209,390	$ (24,256)	$ 270,721	$ 210,959	$ 47,874	$ 231,010	$ 140,925	$ 791,393	$ 666,814	$ 1,211,202	$ 2,859,906
Total benefits and expenses	180,806	193,689	(67,282)	189,338	132,262	182,006	(588,114)	1,163,700	496,551	889,854	2,362,544
Income (loss) from operations before income taxes and equity in earnings of operating joint venture	28,584	(217,945)	338,003	21,621	(84,388)	49,004	729,039	(372,307)	170,263	321,348	497,362
Net income (loss)	$ 113,692	$ (178,496)	$ 349,164	$ 40,072	$ (85,465)	$ 84,897	$ 723,984	$ (328,199)	$ 324,432	$ 395,217	$ 508,003