Income Taxes (Income Tax Provisions) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Examination [Line Items]
Federal income tax expense computed on income at statutory rate	$ 46,283	$ 48,239	$ 55,036
Difference due to:
State taxes, net of federal benefit	936	1,708	1,308
AEFUDC	(2,149)	(1,845)	(2,295)
Permanent tax differences	(1,153)	(604)	(303)
Other	(262)	(519)	(38)
Total income tax expense	$ 43,655	$ 46,979	$ 53,708
Effective income tax rate	33.00%	34.10%	34.20%