Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 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	(380)			(380)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	7,191		5,944	20,860		18,003
Total, pre-tax	(35,017)		(35,316)	(84,020)		(68,968)
Tax benefit	13,004		13,116	31,778		26,148
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(400)			(437)
Losses reclassified from net accumulated other comprehensive loss	20			57
Net current period other comprehensive income (loss)	20			57
Accumulated other comprehensive loss at end of period	(380)			(380)
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			95
Tax benefit	(12)			(38)
Total, net of tax	20			57
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]		$ 95	[1]

[1] Included in interest charges.