Investments (Summary of Investment Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Amortized cost
Available-for-sale	$ 18,052	$ 15,417
Total investment securities	18,672	16,053
Gross unrealized gains
Available-for-sale	275	370
Total investment securities	279	375
Gross unrealized losses
Available-for-sale, Gross unrealized losses	157	56
Total investment securities, Gross unrealized losses	159	57
Estimated fair value
Held-to-maturity, Estimated fair value	622	640
Available-for-sale, Estimated fair value	18,170	15,731
Total investment securities, Estimated fair value	18,792	16,371
Municipal securities
Amortized cost
Held-to-maturity	620	636
Available-for-sale	1,522	1,352
Gross unrealized gains
Held-to-maturity	4	5
Available-for-sale	55	68
Gross unrealized losses
Held-to-maturity, Gross unrealized losses	2	1
Available-for-sale, Gross unrealized losses	3	0
Estimated fair value
Held-to-maturity, Estimated fair value	622	640
Available-for-sale, Estimated fair value	1,574	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, Gross unrealized losses	27	13
Estimated fair value
Available-for-sale, Estimated fair value	128	192
Agency securities
Amortized cost
Available-for-sale	991	1,051
Gross unrealized gains
Available-for-sale	31	40
Gross unrealized losses
Available-for-sale, Gross unrealized losses	0	0
Estimated fair value
Available-for-sale, Estimated fair value	1,022	1,091
Agency guaranteed mortgage-backed securities
Amortized cost
Available-for-sale	14,233	11,439
Gross unrealized gains
Available-for-sale	186	262
Gross unrealized losses
Available-for-sale, Gross unrealized losses	96	8
Estimated fair value
Available-for-sale, Estimated fair value	14,323	11,693
Small Business Administration loan-backed securities
Amortized cost
Available-for-sale	1,076	1,195
Gross unrealized gains
Available-for-sale	3	0
Gross unrealized losses
Available-for-sale, Gross unrealized losses	31	35
Estimated fair value
Available-for-sale, Estimated fair value	1,048	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, Gross unrealized losses	0	0
Estimated fair value
Available-for-sale, Estimated fair value	$ 75	$ 175