Derivative Instruments Derivative Instruments (Fair Value Hedge Attributable to Foreign Currency) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Change in Unrealized Gain (Loss) on Fair Value Hedging Instruments	$ 9,456	$ (1,700)	$ 4,008
Change in Unrealized Gain (Loss) on Hedged Item in Fair Value Hedge	(9,456)	1,700	(4,008)
Gain (Loss) on Fair Value Hedge Ineffectiveness, Net	$ 0	$ 0	$ 0