Derivative Instruments and Hedging Activities Open Foreign Currency Contracts (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Net Unrealized Gain	$ 3,789	$ (426)
Cash Flow Hedging | Foreign Exchange Contract
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	131,859
Net Unrealized Gain	3,010
Cash Flow Hedging | Foreign Exchange Contract | Australian Dollar
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	7,350
Net Unrealized Gain	577
Cash Flow Hedging | Foreign Exchange Contract | Canadian Dollar
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	104,486
Net Unrealized Gain	1,451
Cash Flow Hedging | Foreign Exchange Contract | Mexican Peso
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	20,023
Net Unrealized Gain	$ 982