Income Taxes (Recognized Income Tax Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal:
Current	$ 2,507	$ 2,642	$ 2,319
Deferred	46,089	47,909	32,819
Total federal income tax	48,596	50,551	35,138
State:
Current	(897)	766	1,730
Deferred	1,816	3,285	(1,650)
Total state income tax	919	4,051	80
Generation (amortization) of accumulated investment tax credits	1,489	(684)	(323)
Total income tax expense	$ 51,004	$ 53,918	$ 34,895