Income Taxes - Operating and capital loss carryforwards available (Details) - Dec. 31, 2018 £ in Millions, $ in Millions		USD ($)	GBP (£)
U.S. | Expiring 2019 - 2022
Operating and capital loss carryforwards [Line Items]
Operating loss carryforwards		$ 112
United Kingdom
Operating and capital loss carryforwards [Line Items]
Operating loss carryforwards	[1]	$ 215	£ 168

[1] £168 million