Income Taxes (Income Tax Provisions) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Examination [Line Items]
Federal income tax expense computed on income at statutory rate	$ 40,885	$ 46,381	$ 46,283
Difference due to:
State taxes, net of federal benefit	52	1,902	936
AEFUDC	(2,345)	(3,757)	(2,149)
Permanent tax differences	(2,898)	(2,921)	(1,153)
Other	(799)	(517)	(262)
Total income tax expense	$ 34,895	$ 41,088	$ 43,655
Effective income tax rate	29.90%	31.00%	33.00%