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		$ 2,326,558	$ 2,329,670	$ 2,334,379
Total, pre-tax		(1,704,596)	(1,527,204)	(1,545,121)
Income tax expense (benefit)		581,217	542,719	$ 523,815
Total amounts reclassified, net of tax		(7,179)	(6,362)
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		(3,708)	(2,836)
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		6,050	4,646
Income tax expense (benefit)		(2,342)	(1,810)
Total, net of tax		3,708	2,836
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]	5,859	4,515
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]	191	131
Amortization of net losses | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	[3]	5,912	6,132
Prior service credit | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	[3]	(256)	(357)
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		(3,471)	(3,526)
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		(5,656)	(5,775)
Tax benefit		2,185	2,249
Total amounts reclassified, net of tax		$ 3,471	$ 3,526

[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 9 for details regarding these benefit plans.