Held-to-maturity Securities (Summary of Held-to-maturity Securities in a Continuous Unrealized Loss Position) (Details) $ in Millions	Jun. 30, 2019 USD ($) positions	Dec. 31, 2018 USD ($) positions
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions held less than 12 months | positions	170	150
Number of unrealized loss positions held more than 12 months | positions	70	53
Total number of unrealized loss positions | positions	240	203
Estimated fair value of unrealized loss positions held less than 12 months	$ 9,969	$ 11,484
Estimated fair value of unrealized loss positions held 12 months or more	4,420	2,522
Total estimated fair value of positions in an unrealized loss	14,389	14,006
Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(31)	(49)
Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(29)	(39)
Total gross unrealized loss	$ (60)	$ (88)
State or local housing agency debt obligations
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions held less than 12 months | positions		0
Number of unrealized loss positions held more than 12 months | positions		1
Total number of unrealized loss positions | positions		1
Estimated fair value of unrealized loss positions held less than 12 months		$ 0
Estimated fair value of unrealized loss positions held 12 months or more		1
Total estimated fair value of positions in an unrealized loss		1
Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0
Total gross unrealized loss		$ 0
Government-Sponsored Enterprises Debt Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions held less than 12 months | positions	12	12
Number of unrealized loss positions held more than 12 months | positions	0	1
Total number of unrealized loss positions | positions	12	13
Estimated fair value of unrealized loss positions held less than 12 months	$ 1,204	$ 1,235
Estimated fair value of unrealized loss positions held 12 months or more	0	99
Total estimated fair value of positions in an unrealized loss	1,204	1,334
Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1)	(1)
Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	(1)
Total gross unrealized loss	$ (1)	$ (2)
Mortgage-backed Securities, Private-label residential[Member]
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions held less than 12 months | positions	19	26
Number of unrealized loss positions held more than 12 months | positions	12	11
Total number of unrealized loss positions | positions	31	37
Estimated fair value of unrealized loss positions held less than 12 months	$ 95	$ 139
Estimated fair value of unrealized loss positions held 12 months or more	35	25
Total estimated fair value of positions in an unrealized loss	130	164
Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	(1)
Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(1)	(1)
Total gross unrealized loss	$ (1)	$ (2)
Residential [Member] | Government-sponsored enterprises
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions held less than 12 months | positions	103	54
Number of unrealized loss positions held more than 12 months | positions	19	22
Total number of unrealized loss positions | positions	122	76
Estimated fair value of unrealized loss positions held less than 12 months	$ 2,549	$ 2,704
Estimated fair value of unrealized loss positions held 12 months or more	2,125	1,382
Total estimated fair value of positions in an unrealized loss	4,674	4,086
Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(11)	(13)
Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(19)	(30)
Total gross unrealized loss	$ (30)	$ (43)
Commercial [Member] | Government-sponsored enterprises
Schedule of Held-to-maturity Securities [Line Items]
Number of unrealized loss positions held less than 12 months | positions	36	58
Number of unrealized loss positions held more than 12 months | positions	39	18
Total number of unrealized loss positions | positions	75	76
Estimated fair value of unrealized loss positions held less than 12 months	$ 6,121	$ 7,406
Estimated fair value of unrealized loss positions held 12 months or more	2,260	1,015
Total estimated fair value of positions in an unrealized loss	8,381	8,421
Held-to-maturity, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(19)	(34)
Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(9)	(7)
Total gross unrealized loss	$ (28)	$ (41)