Other Comprehensive Income - Reclassification out of AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		$ (227,629)	$ (234,271)	$ (202,288)
Tax benefit		68,416	82,114	$ 75,025
(Gains) losses reclassified from net accumulated other comprehensive loss		83	139
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	[1]	69	0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Gains) losses reclassified from net accumulated other comprehensive loss		44	0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax		69	0
Reclassification from AOCI, Current Period, Tax		(25)	0
(Gains) losses reclassified from net accumulated other comprehensive loss		44	0
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
(Gains) losses reclassified from net accumulated other comprehensive loss		39	139
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		63	219
Tax benefit		(24)	(80)
Total, net of tax		39	139
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	[2]	$ 63	$ 219

[1] Included in the computation of net periodic pension and postretirement benefit costs. See Note 7 for details regarding these benefit plans.

[2] Included in interest charges.