Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ (23,836)
Other comprehensive income (loss) before reclassifications		(221)
Gains reclassified from net accumulated other comprehensive loss		264
Net current period other comprehensive loss		43
Accumulated other comprehensive loss at end of period		(23,793)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses		178,386	$ 182,417
Total, pre-tax		(183,819)	(176,901)
Tax (benefit) expense		67,945	65,935
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		0
Other comprehensive income (loss) before reclassifications		(219)
Gains reclassified from net accumulated other comprehensive loss		0
Net current period other comprehensive loss		(219)
Accumulated other comprehensive loss at end of period		(219)
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		(23,836)	(23,878)
Other comprehensive income (loss) before reclassifications		(2)	(5)
Gains reclassified from net accumulated other comprehensive loss		264	(105)
Net current period other comprehensive loss		262	(110)
Accumulated other comprehensive loss at end of period		(23,574)	(23,988)
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		427	(169)
Tax (benefit) expense		(163)	64
Total, net of tax		264	(105)
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	[1]	402	(180)
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	[2]	$ 25	$ 11

[1] Included in interest charges.

[2] Included in O&M expenses