Income Taxes - Tax Credit and Loss Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Tax capital loss/credit carryforward	$ 254	$ 268
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4,410	4,112
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	2,228	2,209
Capital loss carryforwards | Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Tax capital loss/credit carryforward	30	$ 58
Alternative minimum tax
Operating Loss Carryforwards [Line Items]
Tax capital loss/credit carryforward	$ 220