Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period				$ (437)
Accumulated other comprehensive loss at end of period	(400)			(400)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	6,814		6,044	13,669		12,059
Total, pre-tax	(16,987)		(9,168)	(49,003)		(33,652)
Tax benefit	6,443		3,426	18,774		13,032
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(418)			(437)
(Gains) losses reclassified from net accumulated other comprehensive loss	18			37
Net current period other comprehensive income (loss)	18			37
Accumulated other comprehensive loss at end of period	(400)			(400)
Gains and Losses on Cash Flow Hedges [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	32			63
Tax benefit	(14)			(26)
Total, net of tax	18			37
Gains and Losses on Cash Flow Hedges [Member] | Interest Rate Derivatives [Member] | Amounts Reclassified from Accumulated Other Comprehensive Loss [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	$ 32	[1]		$ 63	[1]

[1] Included in interest charges.