Income Taxes Operating Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
U.K.
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 675	$ 325
Capital Loss Carryforwards	415	294
U.S. Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	36	193
U.S. State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	412	474
Other
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	392	350
Capital Loss Carryforwards	$ 232	$ 218