Commitments - Extensions of Credit (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Extended lines of credit
Commitments, guarantees:
Unused amounts in lines of credit to third-party entities and commitments for future financing to clients	$ 8,537	$ 8,111
Financing for client purchase agreements
Commitments, guarantees:
Unused amounts in lines of credit to third-party entities and commitments for future financing to clients	$ 3,798	$ 3,569

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