Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	$ (361)
Accumulated other comprehensive income (loss) at end of period	(285)		(361)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(113,045)		(95,877)		(79,509)
Income tax expense (benefit)	42,403		36,409		29,558
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive income (loss) at beginning of period	(361)		(437)
(Gains) losses reclassified from net accumulated other comprehensive loss	76		76
Net current period other comprehensive income (loss)	76		76
Accumulated other comprehensive income (loss) at end of period	(285)		(361)
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	127		127
Income tax expense (benefit)	(51)		(51)
Total, net of tax	76		76
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	$ 127	[1]	$ 127	[1]

[1] Included in interest charges.