Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Accumulated other comprehensive loss at beginning of period				$ (989)
Accumulated other comprehensive loss at end of period		$ (860)		(860)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(98,689)	$ (103,349)	(165,413)	$ (176,294)
Tax benefit		36,874	36,412	60,775	62,587
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		(904)	(1,076)	(989)	(1,161)
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		(860)	(1,032)	(860)	(1,032)
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	[1]	$ 68	$ 68	$ 201	$ 201

[1] Included in interest charges.