Derivative Instruments - Derivatives - Offsetting Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 15	$ 10
Derivative liabilities	(75)	(100)
Other liabilities | Other Investments and Other Liabilities
Derivative [Line Items]
Gross Amounts of Recognized (Liabilities)	(87)	(115)
Gross Amounts Offset in the Statement of Financial Position, liabilities	(11)	(10)
Derivative liabilities	(75)	(100)
Accrued Interest and Cash Collateral (Received)	(1)	(5)
Financial Collateral (Received)	(68)	(96)
Net amount, liabilities	(8)	(9)
Other investments | Other Investments and Other Liabilities
Derivative [Line Items]
Asset Derivatives	27	25
Gross Amounts Offset in the Statement of Financial Position, assets	22	22
Derivative assets	15	10
Financial Collateral Pledged	3	1
Net Amount, assets	2	2
Accrued Interest and Cash Collateral (Received)	$ (10)	$ (7)