Cash and Cash Equivalents (Details) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 USD ($)	Mar. 31, 2015 GBP (£)	Dec. 31, 2014 USD ($)
Cash, Cash Equivalents, and Short-term Investments [Abstract]
Short-term investment, minimum maturity period	3 months	3 months
Short-term investment, maximum maturity period	1 year	1 year
Cash and Cash Equivalents and Short-Term Investments	$ 721		$ 768
Restricted Cash and Investments, Current	127		169
Operating funds in U.K.	60.0	40.5	63.0
Cash and cash equivalents, restricted	$ 67		$ 106