Derivative Instrument (Notional Table) (Details) £ in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 CAD ($)	Mar. 31, 2018 GBP (£)	Dec. 31, 2017 CAD ($)	Dec. 31, 2017 GBP (£)	Mar. 31, 2018 GBP (£)	Dec. 31, 2017 GBP (£)
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivatives Fair Value [Line Items]
Derivative, Notional Amount	$ 18,000		$ 36,000
Designated as Hedging Instrument [Member] | Net Investment Hedging [Member]
Derivatives Fair Value [Line Items]
Derivative, Notional Amount	575,000		575,000		£ 550,000	£ 550,000
Notional Amount Of Nonderivative Instruments	250,000	£ 1,050,000	250,000	£ 1,050,000
Not Designated as Hedging Instrument [Member]
Derivatives Fair Value [Line Items]
Derivative, Notional Amount	$ 408,007		$ 408,007		£ 0	£ 80,000