Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	$ (1,332)
Accumulated other comprehensive loss at end of period	(1,290)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	17,773		16,706
Total, pre-tax	(19,469)		(17,790)
Tax benefit	6,885		6,430
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period	(1,332)
Losses reclassified from net accumulated other comprehensive loss	42
Net current period other comprehensive income	42
Accumulated other comprehensive loss at end of period	(1,290)
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	66
Tax benefit	(24)
Total, net of tax	42
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	$ 66	[1]

[1] Included in interest charges.