Other Comprehensive Income - Reclassification from AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses		$ 762,416		$ 761,901	$ 751,786
Total, pre-tax		(737,409)		(745,242)	(698,779)
Income tax expense (benefit)		273,918		278,440	$ 243,591
Total amounts reclassified, net of tax		(1,059)		(72)
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		(1,056)		(72)
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		1,704		111
Income tax expense (benefit)		(648)		(39)
Total, net of tax		1,056		72
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]	1,618		54
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]	86		57
Amortization of net losses | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		5	[3]	0
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		(3)
Defined Benefit Pension and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(5)		0
Tax Benefit		2		0
Total amounts reclassified, net of tax		$ 3		$ 0

[1] (a) Included in interest charges.

[2] (b) Included in O&M expenses

[3] (c) Included in the computation of net periodic pension and postretirement benefit costs. See Note 8 for details regarding these benefit plans.