Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period					$ (1,161)
Accumulated other comprehensive loss at end of period	(1,032)				(1,032)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	(103,349)		(55,183)		(176,294)		(118,992)
Tax benefit	36,412		20,146		62,587		43,165
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period	(1,076)		(1,247)		(1,161)		(1,332)
Losses reclassified from net accumulated other comprehensive loss	44		44		129		129
Net current period other comprehensive income	44		44		129		129
Accumulated other comprehensive loss at end of period	(1,032)		(1,203)		(1,032)		(1,203)
Gains and Losses on Cash Flow Hedges | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	68		68		201		201
Tax benefit	(24)		(24)		(72)		(72)
Total, net of tax	44		44		129		129
Gains and Losses on Cash Flow Hedges | Interest Rate Derivatives | Amounts Reclassified from Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest charges	$ 68	[1]	$ 68	[1]	$ 201	[1]	$ 201	[1]

[1] Included in interest charges.