Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ (23,878)
Accumulated other comprehensive loss at end of period	(23,988)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	182,417		175,524
Total, pre-tax	(176,901)		(180,900)
Tax expense	65,935		62,497
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(23,878)		(23,338)
Other comprehensive income (loss) before reclassifications	(5)		(3)
Gains reclassified from net accumulated other comprehensive loss	(105)		(120)
Net current period other comprehensive loss	(110)		(123)
Accumulated other comprehensive loss at end of period	(23,988)		(23,461)
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(169)		(193)
Tax expense	64		73
Total, net of tax	(105)		(120)
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	(180)	[1]	(180)	[1]
Gains and Losses on Cash Flow Hedges | Vehicle Fuel Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	$ 11	[2]	$ (13)	[2]

[1] Included in interest charges.

[2] Included in O&M expenses