Income Taxes (Provision for Federal Income Taxes) (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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax expense (benefit)
Federal													$ 14,665	$ 19,144	$ 31,300
State													20	(423)	0
Total current income tax expense													14,685	18,721	31,300
Deferred expense (benefit)
Federal													92,636	23,649	5,700
State													548	442	0
Total deferred income tax expense (benefit)													93,184	24,091	5,700
Change in valuation allowance													(1,089)	(706)	0
Income tax provision	$ 61,234	$ 10,253	$ 23,036	$ 12,257	$ 18,969	$ 2,848	$ 10,532	$ 9,757	$ 10,200	$ 9,600	$ 9,100	$ 8,100	$ 106,780	$ 42,106	$ 37,000