Other Comprehensive Income (Reclassification from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total, pre-tax	$ (40,922)	$ (32,271)	$ (103,107)	$ (91,441)
Tax benefit	9,914	9,946	25,494	32,456
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	32	32	95	94
Tax benefit	(9)	(13)	(26)	(37)
Total, net of tax	23	19	69	57
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	$ 32	$ 32	$ 95	$ 94