Income Taxes (Income Tax Provisions) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Federal income tax expense computed on income at statutory rate	$ 52,740	$ 40,885	$ 46,381
Difference due to:
State taxes, net of federal benefit	2,633	52	1,902
AEFUDC	(475)	(2,345)	(3,757)
Permanent tax differences	(2,369)	(2,898)	(2,921)
Other	1,389	(799)	(517)
Total income tax expense	$ 53,918	$ 34,895	$ 41,088
Effective income tax rate	35.80%	29.90%	31.00%