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				$ (1,332)
Accumulated other comprehensive loss at end of period	(1,247)			(1,247)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	17,844		16,935	35,617		33,641
Total, pre-tax	(44,340)		(47,873)	(63,809)		(65,663)
Tax benefit	16,134		17,579	23,019		24,009
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,290)			(1,332)
(Gains) losses reclassified from net accumulated other comprehensive loss	43			85
Net current period other comprehensive income (loss)	43			85
Accumulated other comprehensive loss at end of period	(1,247)			(1,247)
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	67			133
Tax benefit	(24)			(48)
Total, net of tax	43			85
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	$ 67	[1]		$ 133	[1]

[1] Included in interest charges.