Other Comprehensive Income (Reclassifications from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	$ (96,215)	$ (105,050)	$ (208,491)		$ (199,908)
Tax benefit	14,674	37,269	35,400		71,710
Total amounts reclassified, net of tax	31	26	92		75
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	13	10	37		29
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	16	47		47
Tax benefit	(3)	(6)	(10)		(18)
Total, net of tax	13	10	37		29
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	16	16	47		47	[1]
Amortization of net loss | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	23	24	70	[2]	72	[2]
Defined Benefit Pension and Postretirement Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total amounts reclassified, net of tax	18	16	55		46
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	23	24	70		72
Tax benefit	(5)	(8)	(15)		(26)
Total amounts reclassified, net of tax	$ 18	$ 16	$ 55		$ 46

[1] Included in interest charges.

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