Derivative Instruments and Hedging Activities Open Foreign Currency Contracts (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Net Unrealized Gain	$ 5,931	$ (426)
Cash Flow Hedging | Foreign Exchange Contract
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	170,470
Net Unrealized Gain	5,931
Cash Flow Hedging | Foreign Exchange Contract | Australian Dollar
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	17,658
Net Unrealized Gain	985
Cash Flow Hedging | Foreign Exchange Contract | Canadian Dollar
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	132,052
Net Unrealized Gain	5,336
Cash Flow Hedging | Foreign Exchange Contract | Mexican Peso
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	20,760
Net Unrealized Gain	$ (390)