Income Taxes (Provision for Federal Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax expense (benefit)
Federal													$ (653)	$ 14,665	$ 19,144
State													321	20	(423)
Total current income tax expense													(332)	14,685	18,721
Deferred income tax expense
Federal													41,687	92,636	23,649
State													229	548	442
Total deferred income tax expense													41,916	93,184	24,091
Change in valuation allowance													317	(1,089)	(706)
Income tax provision	$ 5,200	$ 11,312	$ 12,434	$ 12,955	$ 61,234	$ 10,253	$ 23,036	$ 12,257	$ 18,969	$ 2,848	$ 10,532	$ 9,757	$ 41,901	$ 106,780	$ 42,106