Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ 1,807,949
Accumulated other comprehensive loss at end of period		1,931,696	$ 1,807,949
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(234,271)	(202,288)	$ (205,054)
Tax benefit		82,114	75,025	75,202
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(1,281)	(989)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(148)	(464)
(Gains) losses reclassified from net accumulated other comprehensive loss		139	172
Net current period other comprehensive income (loss)		(9)	(292)
Accumulated other comprehensive loss at end of period		(1,290)	(1,281)	(989)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(464)	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(148)	(464)
(Gains) losses reclassified from net accumulated other comprehensive loss		0	0
Net current period other comprehensive income (loss)		(148)	(464)
Accumulated other comprehensive loss at end of period		(612)	(464)	0
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(817)	(989)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0	0
(Gains) losses reclassified from net accumulated other comprehensive loss		139	172
Net current period other comprehensive income (loss)		139	172
Accumulated other comprehensive loss at end of period		(678)	(817)	$ (989)
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		219	269
Tax benefit		(80)	(97)
Total, net of tax		139	172
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]	$ 219	$ 269

[1] (a) Included in interest charges.