Other Comprehensive Income (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period		$ 790,385
Accumulated other comprehensive loss at end of period		811,850	$ 790,385
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax		(112,037)	(118,874)	$ (113,045)
Income tax expense (benefit)		42,902	44,238	42,403
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss at beginning of period		(209)	(285)
Accumulated other comprehensive loss at end of period		(133)	(209)	$ (285)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
(Gains) losses reclassified from net accumulated other comprehensive loss		76	76
Net current period other comprehensive income (loss)		76	76
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		127	127
Income tax expense (benefit)		(51)	(51)
Total, net of tax		76	76
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]	$ 127	$ 127

[1] Included in interest charges.