Derivative Financial Instruments Outstanding British Pound Forward Contracts (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2017 GBP (£)
British Pound Foreign Currency Forward Contract 1 [Member]
Effective Date	Mar. 25, 2015
Expiration Date	Mar. 31, 2020
Notional Amount	£ 60,000
Derivative, Forward Exchange Rate	1.51
British Pound Foreign Currency Forward Contract 2 [Member]
Effective Date	Mar. 25, 2015
Expiration Date	Mar. 31, 2020
Notional Amount	£ 60,000
Derivative, Forward Exchange Rate	1.50
British Pound Foreign Currency Forward Contract 3 [Member]
Effective Date	Jun. 30, 2015
Expiration Date	Mar. 31, 2020
Notional Amount	£ 2,035
Derivative, Forward Exchange Rate	1.57
British Pound Foreign Currency Forward Contract 4 [Member]
Effective Date	Sep. 30, 2015
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,846
Derivative, Forward Exchange Rate	1.51
British Pound Foreign Currency Forward Contract 5 [Member]
Effective Date	Dec. 31, 2015
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,873
Derivative, Forward Exchange Rate	1.48
British Pound Foreign Currency Forward Contract 6 [Member]
Effective Date	Mar. 31, 2016
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,881
Derivative, Forward Exchange Rate	1.45
British Pound Foreign Currency Forward Contract 7 [Member]
Effective Date	Jun. 30, 2016
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,909
Derivative, Forward Exchange Rate	1.35
British Pound Foreign Currency Forward Contract 8 [Member]
Effective Date	Sep. 30, 2016
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,959
Derivative, Forward Exchange Rate	1.33
British Pound Foreign Currency Forward Contract 9 [Member]
Effective Date	Dec. 30, 2016
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,989
Derivative, Forward Exchange Rate	1.28
British Pound Foreign Currency Forward Contract 10 [Member]
Effective Date	Mar. 31, 2017
Expiration Date	Mar. 31, 2020
Notional Amount	£ 1,975
Derivative, Forward Exchange Rate	1.30
British Pound Foreign Currency Forward Contract 11 [Member]
Effective Date	Jun. 30, 2017
Expiration Date	Mar. 31, 2020
Notional Amount	£ 2,026
Derivative, Forward Exchange Rate	1.34
British Pound Foreign Currency Forward Contract 12 [Member]
Effective Date	Oct. 02, 2017
Expiration Date	Mar. 31, 2020
Notional Amount	£ 2,079
Derivative, Forward Exchange Rate	1.36
British Pound Foreign Currency Forward Contract 13 [Member]
Effective Date	Dec. 29, 2017
Expiration Date	Mar. 31, 2020
Notional Amount	£ 2,111
Derivative, Forward Exchange Rate	1.39