Income Taxes (Components of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current tax expense (benefit):
U.S. Federal		$ 36,739	$ (52,502)	$ (99,443)
State and local		0	0	49
Total		36,739	(52,502)	(99,394)
Deferred tax expense (benefit):
U.S. Federal		(89,380)	(104,326)	30,902
Total		(89,380)	(104,326)	30,902
Total income tax expense (benefit)	$ (2,699)	(52,641)	(156,828)	(68,492)
Total income tax expense (benefit) reported in equity related to:
Other comprehensive income (loss)		(55,174)	43,372	3,322
Additional paid-in capital		0	824	587
Total income tax expense (benefit)		$ (107,815)	$ (112,632)	$ (64,583)