Investments (Summary of Investment Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Amortized cost
Available-for-sale	$ 20,473	$ 15,417
Total HTM and AFS investment securities	20,932	16,053
Gross unrealized gains
Available-for-sale	225	370
Total HTM and AFS investment securities	229	375
Gross unrealized losses
Available-for-sale	237	56
Total HTM and AFS investment securities	239	57
Estimated fair value
Held-to-maturity, Estimated fair value	461	640
Available-for-sale	20,461	15,731
Total HTM and AFS investment securities	20,922	16,371
Municipal securities
Amortized cost
Held-to-maturity	459	636
Available-for-sale	1,513	1,352
Gross unrealized gains
Held-to-maturity	4	5
Available-for-sale	48	68
Gross unrealized losses
Held-to-maturity	2	1
Available-for-sale	5	0
Estimated fair value
Held-to-maturity, Estimated fair value	461	640
Available-for-sale	1,556	1,420
U.S. Treasury securities
Amortized cost
Available-for-sale	155	205
Gross unrealized gains
Available-for-sale	0	0
Gross unrealized losses
Available-for-sale	27	13
Estimated fair value
Available-for-sale	128	192
Agency securities
Amortized cost
Available-for-sale	874	1,051
Gross unrealized gains
Available-for-sale	21	40
Gross unrealized losses
Available-for-sale	0	0
Estimated fair value
Available-for-sale	895	1,091
Agency guaranteed mortgage-backed securities
Amortized cost
Available-for-sale	16,841	11,439
Gross unrealized gains
Available-for-sale	153	262
Gross unrealized losses
Available-for-sale	175	8
Estimated fair value
Available-for-sale	16,819	11,693
Small Business Administration loan-backed securities
Amortized cost
Available-for-sale	1,015	1,195
Gross unrealized gains
Available-for-sale	3	0
Gross unrealized losses
Available-for-sale	30	35
Estimated fair value
Available-for-sale	988	1,160
Other debt securities
Amortized cost
Available-for-sale	75	175
Gross unrealized gains
Available-for-sale	0	0
Gross unrealized losses
Available-for-sale	0	0
Estimated fair value
Available-for-sale	$ 75	$ 175