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 income (loss) at beginning of period	$ (21,081)	$ (20,814)
Other comprehensive income (loss) before reclassifications	(1)	(5)
(Gains) losses reclassified from net accumulated other comprehensive loss	242	202
Net current period other comprehensive income (loss)	241	197
Accumulated other comprehensive income (loss) at end of period	(20,840)	(20,617)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses	320,496	314,050
Total, pre-tax	(141,097)	(9,894)
Tax benefit	46,468	2,970
Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax	242	202
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(19,090)	(19,909)
Other comprehensive income (loss) before reclassifications	(1)	(6)
(Gains) losses reclassified from net accumulated other comprehensive loss	223	208
Net current period other comprehensive income (loss)	222	202
Accumulated other comprehensive income (loss) at end of period	(18,868)	(19,707)
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	377	351
Tax benefit	(154)	(143)
Total, net of tax	223	208
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	346	337
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	31	14
Unrealized Gains and Losses on Marketable Securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	105	105
Other comprehensive income (loss) before reclassifications	0	1
(Gains) losses reclassified from net accumulated other comprehensive loss	0	0
Net current period other comprehensive income (loss)	0	1
Accumulated other comprehensive income (loss) at end of period	105	106
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(2,096)	(1,010)
Other comprehensive income (loss) before reclassifications	0	0
(Gains) losses reclassified from net accumulated other comprehensive loss	19	(6)
Net current period other comprehensive income (loss)	19	(6)
Accumulated other comprehensive income (loss) at end of period	(2,077)	(1,016)
Defined Benefit Pension and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net loss	83	39
Prior service credit	(49)	(49)
Total, pre-tax	34	(10)
Tax benefit	(15)	4
Total, net of tax	$ 19	$ (6)