Borrowings - Securities Sold Under Agreements to Repurchase (Narrative) (Details) - Securities sold under agreements to repurchase - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Percentage of collateral in residential mortgage-backed securities	82.00%	82.00%
Percentage of collateral in obligations of GSEs	18.00%	18.00%
Securities pledged as collateral for repurchase agreements, amortized cost	$ 1,230	$ 1,210
Securities pledged as collateral for repurchase agreements, fair value	$ 1,220	$ 1,200
Minimum
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Debt instrument, term	4 years	4 years
Maximum
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Debt instrument, term	7 years	7 years

Debt Instrument, Collateral, Percentage of Collateral in Obligations of Government-Sponsored Enterprises

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- Definition

Debt Instrument, Collateral, Percentage of Collateral in Residential Mortgage-backed Securities

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No definition available.

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- Definition

The amortized cost, as of the date of each statement of financial position presented, including accrued interest, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

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No definition available.

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- Definition

The fair value, including accrued interest, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

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No definition available.

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- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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