Income Taxes (Income Tax Provisions) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Examination [Line Items]
Federal income tax expense computed on income at statutory rate	$ 52,243	$ 52,740	$ 40,885
Difference due to:
State taxes, net of federal benefit	597	2,633	52
AEFUDC	450	(475)	(2,345)
Permanent tax differences	(2,562)	(2,369)	(2,898)
Other	276	1,389	(799)
Total income tax expense	$ 51,004	$ 53,918	$ 34,895
Effective income tax rate	34.20%	35.80%	29.90%