Other Comprehensive Income (Reclassification from AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Operating and maintenance expenses		$ 173,905	$ 191,011	$ 547,413	$ 570,343
Total, pre-tax		(290,375)	(276,823)	(624,144)	(601,215)
Tax (benefit) expense		104,298	103,216	225,934	224,390
Total amounts reclassified, net of tax		258	266	756	792
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		257	266	753	792
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		407	428	1,208	1,278
Tax (benefit) expense		(150)	(162)	(455)	(486)
Total, net of tax		257	266	753	792
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]	407	407	1,208	1,211
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]	0	21	0	67
Amortizaton of net loss | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	[3]	2	0	6	0
Defined Benefit and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		1	0	3	2
Defined Benefit and Postretirement Items | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(2)	0	6	0
Tax benefit		(1)	0	(3)	0
Total amounts reclassified, net of tax		$ (1)	$ 0	$ 3	$ 0

[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 11 for details regarding these benefit plans.