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		$ (108,139)	$ (106,275)
Other comprehensive income (loss) before reclassifications		(7,976)	(7,647)
(Gains) losses reclassified from net accumulated other comprehensive loss		6,362	5,783
Net current period other comprehensive income (loss)		(1,614)	(1,864)
Accumulated other comprehensive income (loss) at end of period		(109,753)	(108,139)	$ (106,275)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses		2,329,670	2,334,379	2,273,532
Total, pre-tax		(1,527,204)	(1,545,121)	(1,432,210)
Income tax expense (benefit)		542,719	523,815	483,976
Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, net of tax		6,362	5,783
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		(57,628)	(59,753)
Other comprehensive income (loss) before reclassifications		(70)	(163)
(Gains) losses reclassified from net accumulated other comprehensive loss		2,836	2,288
Net current period other comprehensive income (loss)		2,766	2,125
Accumulated other comprehensive income (loss) at end of period		(54,862)	(57,628)	(59,753)
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		4,646	3,781
Income tax expense (benefit)		(1,810)	(1,493)
Total, net of tax		2,836	2,288
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]	4,515	3,836
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]	131	(55)
Unrealized Gains and Losses on Marketable Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		110	77
Other comprehensive income (loss) before reclassifications		0	33
(Gains) losses reclassified from net accumulated other comprehensive loss		0	0
Net current period other comprehensive income (loss)		0	33
Accumulated other comprehensive income (loss) at end of period		110	110	77
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		(50,621)	(46,599)
Other comprehensive income (loss) before reclassifications		(7,906)	(7,517)
(Gains) losses reclassified from net accumulated other comprehensive loss		3,526	3,495
Net current period other comprehensive income (loss)		(4,380)	(4,022)
Accumulated other comprehensive income (loss) at end of period		(55,001)	(50,621)	$ (46,599)
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	[3]	6,132	5,998
Prior service (credit) cost	[3]	(357)	(344)
Total, pre-tax		5,775	5,654
Income tax expense (benefit)		(2,249)	(2,159)
Total, net of tax		$ 3,526	$ 3,495

[1] Included in interest charges.

[2] Included in O&M expenses.

[3] Included in the computation of net periodic pension and postretirement benefit costs. See Note 9 for details regarding these benefit plans.