Income Taxes - Operating and capital loss carryforwards available (Details) - Dec. 31, 2016 £ in Millions, $ in Millions		USD ($)	GBP (£)
U.S. | Expiring 2017 - 2022
Operating and capital loss carryforwards [Line Items]
Operating loss carryforwards		$ 143
U.S. | Capital loss carryforward | Expiring 2020
Operating and capital loss carryforwards [Line Items]
Capital loss carryforwards		237
United Kingdom
Operating and capital loss carryforwards [Line Items]
Operating loss carryforwards	[1]	$ 139	£ 112

[1] £112 million