Other Comprehensive Income (Reclassifications from AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		$ (55,676)	$ (50,479)	$ (94,858)	$ (85,814)
Tax benefit		20,314	18,268	34,441	31,080
Total amounts reclassified, net of tax		25	43	49	85
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		10	43	19	85
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		16	67	31	134
Tax benefit		(6)	(24)	(12)	(49)
Total, net of tax		10	43	19	85
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]	16	67	31	134
Amortization of net loss | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	[2]	24	0	48	0
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax		15	0	30	0
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		24	0	48	0
Tax benefit		(9)	0	(18)	0
Total amounts reclassified, net of tax		$ 15	$ 0	$ 30	$ 0

[1] Included in interest charges.

[2] Included in the computation of net periodic pension and postretirement benefit costs. See Note 10 for details regarding these benefit plans.