Cash and Cash Equivalents and Short-term Investments (Details) £ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 GBP (£)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Cash and cash equivalents and short-term investments			$ 734.0	$ 828.0
Cash and cash equivalents and short term investments, period increase (decrease)	$ (94.0)
Restricted cash and investments			140.0	91.0
Restricted cash and cash equivalents			48.0
Operating funds in U.K.		£ 42.7	$ 56.4	$ 53.9