Collateral and Offsetting of Assets and Liabilities - Underlying Collateral Pledged (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Collateral in the Form of Securities Pledged [Line Items]
Securities pledged with the ability for the secured party to repledge	$ 7,450	$ 13,131
Securities Sold under Agreements to Repurchase
Collateral in the Form of Securities Pledged [Line Items]
Securities pledged with the ability for the secured party to repledge	2,665	$ 9,346
Securities Sold under Agreements to Repurchase | Overnight and continuous
Collateral in the Form of Securities Pledged [Line Items]
Securities pledged with the ability for the secured party to repledge	2,665
Securities Sold under Agreements to Repurchase | 30 days or less
Collateral in the Form of Securities Pledged [Line Items]
Securities pledged with the ability for the secured party to repledge	0
Securities Sold under Agreements to Repurchase | After 30 days through 90 days
Collateral in the Form of Securities Pledged [Line Items]
Securities pledged with the ability for the secured party to repledge	0
Securities Sold under Agreements to Repurchase | Greater than 90 days
Collateral in the Form of Securities Pledged [Line Items]
Securities pledged with the ability for the secured party to repledge	$ 0