Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		$ (1,281)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		12
Losses reclassified from net accumulated other comprehensive loss		42
Net current period other comprehensive income		54
Accumulated other comprehensive loss at end of period		(1,227)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(35,335)	$ (31,585)
Tax benefit		12,812	11,338
Defined Benefit Pension and Postretirement Items
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period		(464)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		12
Losses reclassified from net accumulated other comprehensive loss		0
Net current period other comprehensive income		12
Accumulated other comprehensive loss at end of period		(452)
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), 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
Losses reclassified from net accumulated other comprehensive loss		42	42
Net current period other comprehensive income		42	42
Accumulated other comprehensive loss at end of period		(775)	(947)
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		67	66
Tax benefit		(25)	(24)
Total, net of tax		42	42
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]	$ 67	$ 66

[1] Included in interest charges.