Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		$ (23,878)
Accumulated other comprehensive income (loss) at end of period		(23,836)	$ (23,878)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses		761,901	751,786	$ 762,322
Total, pre-tax		(745,242)	(698,779)	(704,123)
Income tax expense (benefit)		278,440	243,591	250,740
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		(23,878)	(23,338)
Other comprehensive income (loss) before reclassifications		(30)	(72)
(Gains) losses reclassified from net accumulated other comprehensive loss		72	(468)
Net current period other comprehensive income (loss)		42	(540)
Accumulated other comprehensive income (loss) at end of period		(23,836)	(23,878)	$ (23,338)
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		111	(755)
Income tax expense (benefit)		(39)	287
Total, net of tax		72	(468)
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]	54	(730)
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]	$ 57	$ (25)

[1] Included in interest charges.

[2] Included in O&M expenses