Income Taxes (Components of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current tax expense (benefit):
U.S. Federal				$ (99,443)	$ 105,992	$ 112,742
State and local				49	129	0
Total				(99,394)	106,121	112,742
Deferred tax expense (benefit):
U.S. Federal				25,525	(116,762)	24,327
Total				25,525	(116,762)	24,327
Total income tax expense (benefit)	$ 5,055	$ (44,108)	$ (39,053)	(73,869)	(10,641)	137,069
Total income tax expense (benefit) reported in equity related to:
Other comprehensive income (loss)				3,322	(61,322)	65,717
Additional paid-in capital				587	(6,560)	(6,507)
Total income tax expense (benefit)				$ (69,960)	$ (78,523)	$ 196,279