Securities with Unrealized Losses (Detail) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017		Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 11,115	[1]	$ 19,459	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	105	[3]	383	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,299	[1]	464	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	121	[3]	9	[4]
Total, Fair Value	15,414	[1]	19,923	[2]
Total, Unrealized Losses	226	[3]	392	[4]
Temporarily Impaired Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	11,101		19,443
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	104	[3]	383	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	4,298		463
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	121	[3]	9	[4]
Total, Fair Value	15,399		19,906
Total, Unrealized Losses	225	[3]	392	[4]
Temporarily Impaired Securities [Member] | US Treasury Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	1,092		4,380
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	9	[3]	86	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,382		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	39	[3]	0	[4]
Total, Fair Value	2,474		4,380
Total, Unrealized Losses	48	[3]	86	[4]
Temporarily Impaired Securities [Member] | US Government Agencies Debt Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	43		96
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	[3]	2	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	33		3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	1	[3]	0	[4]
Total, Fair Value	76		99
Total, Unrealized Losses	1	[3]	2	[4]
Temporarily Impaired Securities [Member] | US States and Political Subdivisions Debt Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	178		149
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	1	[3]	5	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	119		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	3	[3]	0	[4]
Total, Fair Value	297		149
Total, Unrealized Losses	4	[3]	5	[4]
Temporarily Impaired Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	9,571		14,622
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	92	[3]	285	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,709		451
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	75	[3]	8	[4]
Total, Fair Value	12,280		15,073
Total, Unrealized Losses	167	[3]	293	[4]
Temporarily Impaired Securities [Member] | Commercial Mortgage Backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	207		184
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	2	[3]	5	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	47		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	1	[3]	0	[4]
Total, Fair Value	254		184
Total, Unrealized Losses	3	[3]	5	[4]
Temporarily Impaired Securities [Member] | Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	[3]	0	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5		5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	[3]	0	[4]
Total, Fair Value	5		5
Total, Unrealized Losses	0	[3]	0	[4]
Temporarily Impaired Securities [Member] | Other Debt Obligations [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	10		12
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	[3]	0	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	[3]	0	[4]
Total, Fair Value	10		12
Total, Unrealized Losses	0	[3]	0	[4]
Temporarily Impaired Securities [Member] | Equity Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	[3]	0	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	3		4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	2	[3]	1	[4]
Total, Fair Value	3		4
Total, Unrealized Losses	2	[3]	1	[4]
Other Than Temporarily Impaired Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	14	[1]	16	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	1	[3]	0	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1	[1]	1	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	[3]	0	[4]
Total, Fair Value	15	[1]	17	[2]
Total, Unrealized Losses	1	[3]	0	[4]
Other Than Temporarily Impaired Securities [Member] | Asset-backed Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	[1]	0	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	0	[3]	0	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1	[1]	1	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	[3]	0	[4]
Total, Fair Value	1	[1]	1	[2]
Total, Unrealized Losses	0	[3]	0	[4]
Other Than Temporarily Impaired Securities [Member] | Mortgage-backed Securities, Issued by Private Enterprises [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	14	[1]	16	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	1	[3]	0	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	[1]	0	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0	[3]	0	[4]
Total, Fair Value	14	[1]	16	[2]
Total, Unrealized Losses	$ 1	[3]	$ 0	[4]

[1] OTTI securities AFS are impaired securities for which OTTI credit losses have been previously recognized in earnings.

[2] OTTI securities AFS are impaired securities for which OTTI credit losses have been previously recognized in earnings.

[3] Unrealized losses less than $0.5 million are presented as zero within the table.

[4] Unrealized losses less than $0.5 million are presented as zero within the table.