Income Taxes (Components of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax expense (benefit):
U.S. Federal	$ (52,501)	$ (99,443)	$ 105,992
State and local	0	49	129
Total	(52,501)	(99,394)	106,121
Deferred tax expense (benefit):
U.S. Federal	(102,153)	25,525	(116,762)
Total	(102,153)	25,525	(116,762)
Total income tax expense (benefit)	(154,654)	(73,869)	(10,641)
Total income tax expense (benefit) reported in equity related to:
Other comprehensive income (loss)	43,372	3,322	(61,322)
Additional paid-in capital	824	587	(6,560)
Total income tax expense (benefit)	$ (110,458)	$ (69,960)	$ (78,523)