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				$ (1,332)
Accumulated other comprehensive loss at end of period	(1,203)			(1,203)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	22,892		17,649	58,509		51,290
Total, pre-tax	(55,183)		(92,509)	(118,992)		(158,172)
Tax benefit	20,146		34,262	43,165		58,271
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,247)			(1,332)
Losses reclassified from net accumulated other comprehensive loss	44			129
Net current period other comprehensive income (loss)	44			129
Accumulated other comprehensive loss at end of period	(1,203)			(1,203)
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	68			201
Tax benefit	(24)			(72)
Total, net of tax	44			129
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	$ 68	[1]		$ 201	[1]

[1] Included in interest charges.