Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	$ (1,161)
Accumulated other comprehensive loss at end of period	(1,118)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(29,103)		(19,469)
Tax benefit	10,368		6,885
Gains and Losses on Cash Flow Hedges [Member]
Accumulated Other Comprehensive Income [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,161)		(1,332)
Losses reclassified from net accumulated other comprehensive loss	43		42
Net current period other comprehensive income	43		42
Accumulated other comprehensive loss at end of period	(1,118)		(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	67		66
Tax benefit	(24)		(24)
Total, net of tax	43		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 Income (Expense), Net	$ 67	[1]	$ 66	[1]

[1] Included in interest charges.