Fair Value Measurement - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financial assets:
Loans held for sale	$ 1,700	$ 596
Level 1
Financial assets:
Cash and cash equivalents	5,444	4,708
Restricted cash for securitization investors	234	262
Net loans held for investment	0	0
Loans held for sale	0	0
Interest receivable	0	0
Other investments	0	0
Financial liabilities:
Deposits with defined maturities	0	0
Securitized debt obligations	0	0
Senior and subordinated notes	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase	0	0
Interest payable	0	0
Level 2
Financial assets:
Cash and cash equivalents	21,180	35,801
Restricted cash for securitization investors	0	0
Net loans held for investment	0	0
Loans held for sale	4,880	2,214
Interest receivable	1,418	1,471
Other investments	1,344	1,341
Financial liabilities:
Deposits with defined maturities	19,483	33,111
Securitized debt obligations	12,782	12,584
Senior and subordinated notes	24,754	28,282
Federal funds purchased and securities loaned or sold under agreements to repurchase	825	668
Interest payable	241	352
Level 3
Financial assets:
Cash and cash equivalents	0	0
Restricted cash for securitization investors	0	0
Net loans held for investment	254,878	244,701
Loans held for sale	0	0
Interest receivable	0	0
Other investments	0	0
Financial liabilities:
Deposits with defined maturities	0	0
Securitized debt obligations	0	0
Senior and subordinated notes	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase	0	0
Interest payable	0	0
Carrying Value
Financial assets:
Cash and cash equivalents	26,624	40,509
Restricted cash for securitization investors	234	262
Net loans held for investment	249,817	236,060
Loans held for sale	4,593	2,114
Interest receivable	1,418	1,471
Other investments	1,344	1,341
Financial liabilities:
Deposits with defined maturities	19,283	32,746
Securitized debt obligations	12,635	12,414
Senior and subordinated notes	23,983	27,382
Federal funds purchased and securities loaned or sold under agreements to repurchase	825	668
Interest payable	241	352
Estimated Fair Value
Financial assets:
Cash and cash equivalents	26,624	40,509
Restricted cash for securitization investors	234	262
Net loans held for investment	254,878	244,701
Loans held for sale	4,880	2,214
Interest receivable	1,418	1,471
Other investments	1,344	1,341
Financial liabilities:
Deposits with defined maturities	19,483	33,111
Securitized debt obligations	12,782	12,584
Senior and subordinated notes	24,754	28,282
Federal funds purchased and securities loaned or sold under agreements to repurchase	825	668
Interest payable	$ 241	$ 352