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Cash and Cash Equivalents and Short-term Investments (Details)
£ in Millions, $ in Millions
Mar. 31, 2017
GBP (£)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
GBP (£)
Dec. 31, 2016
USD ($)
Cash, Cash Equivalents, and Short-term Investments [Abstract]        
Cash and cash equivalents and short-term investments   $ 633.0   $ 716.0
Restricted cash   86.0   82.0
Operating funds in U.K. £ 43.3 $ 54.0 £ 43.3 $ 53.2