Derivative Instruments (Current Period Cash Flow Hedges in AOCI (loss) before Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss)
Beginning Balance	$ 11,745	$ 14,847	$ 4,839
Net deferred gains (losses) on cash flow hedges for the period	(39,434)	9,698	12,078
Amount reclassified into current period earnings	1,838	(12,800)	(2,070)
Ending Balance	$ (25,851)	$ 11,745	$ 14,847