Investments (Summary of Investment Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Marketable Securities [Line Items]
Available-for-sale, Amortized cost	$ 15,417	$ 13,686
Total investment securities, Amortized cost	16,053	14,278
Available-for-sale, Gross unrealized gains	370	126
Total investment securities, Gross unrealized gains	375	131
Available-for-sale, Gross unrealized losses	56	87
Total investment securities, Gross unrealized losses	57	87
Held-to-maturity investment securities	640	597
Available-for-sale, at fair value	15,731	13,725
Total investment securities, Estimated fair value	16,371	14,322
Municipal securities
Marketable Securities [Line Items]
Held-to-maturity, Amortized cost	636	592
Available-for-sale, Amortized cost	1,352	1,282
Held-to-maturity, Gross unrealized gains	5	5
Available-for-sale, Gross unrealized gains	68	37
Held-to-maturity, Gross unrealized losses	1	0
Available-for-sale, Gross unrealized losses	0	0
Held-to-maturity investment securities	640	597
Available-for-sale, at fair value	1,420	1,319
U.S. Treasury securities
Marketable Securities [Line Items]
Available-for-sale, Amortized cost	205	25
Available-for-sale, Gross unrealized gains	0	0
Available-for-sale, Gross unrealized losses	13	0
Available-for-sale, at fair value	192	25
Agency securities
Marketable Securities [Line Items]
Available-for-sale, Amortized cost	1,051	1,301
Available-for-sale, Gross unrealized gains	40	5
Available-for-sale, Gross unrealized losses	0	4
Available-for-sale, at fair value	1,091	1,302
Agency guaranteed mortgage-backed securities
Marketable Securities [Line Items]
Available-for-sale, Amortized cost	11,439	9,518
Available-for-sale, Gross unrealized gains	262	83
Available-for-sale, Gross unrealized losses	8	42
Available-for-sale, at fair value	11,693	9,559
Small Business Administration loan-backed securities
Marketable Securities [Line Items]
Available-for-sale, Amortized cost	1,195	1,535
Available-for-sale, Gross unrealized gains	0	1
Available-for-sale, Gross unrealized losses	35	41
Available-for-sale, at fair value	1,160	1,495
Other debt securities
Marketable Securities [Line Items]
Available-for-sale, Amortized cost	175	25
Available-for-sale, Gross unrealized gains	0	0
Available-for-sale, Gross unrealized losses	0	0
Available-for-sale, at fair value	$ 175	$ 25

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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