Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period					$ (1,161)
Accumulated other comprehensive loss at end of period	(1,076)				(1,076)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(43,842)		(44,340)		(72,945)		(63,809)
Tax benefit	15,807		16,134		26,175		23,019
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,118)		(1,290)		(1,161)		(1,332)
Losses reclassified from net accumulated other comprehensive loss	42		43		85		85
Net current period other comprehensive income	42		43		85		85
Accumulated other comprehensive loss at end of period	(1,076)		(1,247)		(1,076)		(1,247)
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	66		67		133		133
Tax benefit	(24)		(24)		(48)		(48)
Total, net of tax	42		43		85		85
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	$ 66	[1]	$ 67	[1]	$ 133	[1]	$ 133	[1]

[1] Included in interest charges.