Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period				$ (285)
Accumulated other comprehensive loss at end of period		$ (228)		(228)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(41,867)	$ (32,139)	(97,173)	$ (89,606)
Tax benefit		15,635	12,109	36,162	33,319
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		(247)	(324)	(285)	(361)
Losses reclassified from net accumulated other comprehensive loss		19	20	57	57
Net current period other comprehensive income		19	20	57	57
Accumulated other comprehensive loss at end of period		(228)	(304)	(228)	(304)
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		32	32	94	95
Tax benefit		(13)	(12)	(37)	(38)
Total, net of tax		19	20	57	57
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]	$ 32	$ 32	$ 94	$ 95

[1] Included in interest charges.