Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		$ (989)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(464)
(Gains) losses reclassified from net accumulated other comprehensive loss		172
Net current period other comprehensive income (loss)		(292)
Accumulated other comprehensive income (loss) at end of period		(1,281)	$ (989)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(202,288)	(205,054)	$ (148,938)
Tax benefit		75,025	75,202	53,761
Gains and Losses on Cash Flow Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		(989)	(1,161)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0
(Gains) losses reclassified from net accumulated other comprehensive loss		172	172
Net current period other comprehensive income (loss)		172	172
Accumulated other comprehensive income (loss) at end of period		(817)	(989)	$ (1,161)
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		269	268
Tax benefit		(97)	(96)
Total, net of tax		172	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]	269	268
Defined Benefit Pension and Postretirement Items
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period		0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(464)
(Gains) losses reclassified from net accumulated other comprehensive loss		0
Net current period other comprehensive income (loss)		(464)
Accumulated other comprehensive income (loss) at end of period		$ (464)	$ 0

[1] Included in interest charges.