Derivative Instruments and Hedging Activities Open Foreign Currency Contracts (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Net Unrealized Gain	$ 5,264	$ (426)
Cash Flow Hedging | Foreign Exchange Contract
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	198,247
Net Unrealized Gain	5,264
Cash Flow Hedging | Foreign Exchange Contract | Australian Dollar
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	26,496
Net Unrealized Gain	470
Cash Flow Hedging | Foreign Exchange Contract | Canadian Dollar
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	154,671
Net Unrealized Gain	3,796
Cash Flow Hedging | Foreign Exchange Contract | Mexican Peso
Derivative [Line Items]
Notional Amounts (in U.S. Dollars)	17,080
Net Unrealized Gain	$ 998