Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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		$ (247)		(247)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(19,975)	$ (19,309)	(55,306)	$ (57,467)
Tax benefit		7,463	7,287	20,527	21,210
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		(266)	(343)	(285)	(361)
Losses reclassified from net accumulated other comprehensive loss		19	19	38	37
Net current period other comprehensive income		19	19	38	37
Accumulated other comprehensive loss at end of period		(247)	(324)	(247)	(324)
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		31	31	63	62
Tax benefit		(12)	(12)	(25)	(25)
Total, net of tax		19	19	38	37
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]	$ 31	$ 31	$ 63	$ 62

[1] Included in interest charges.