Securities with Unrealized Losses (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019		Dec. 31, 2018
Investments, Unrealized Loss Position [Line Items]
Document Period End Date		Sep. 30, 2019
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		$ 1,485		$ 1,452
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		6	[1]	7	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		6		22,822
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	603	[2]
Total, Fair Value		1,491		24,274
Total, Unrealized Losses		6	[1]	610	[2]
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		0	[3]	0	[4]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[1],[3]	0	[2],[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0	[3]	0	[4]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1],[3]	0	[2],[4]
Total, Fair Value		0	[3]	0	[4]
Total, Unrealized Losses		0	[1],[3]	0	[2],[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		1,485		1,452
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		6	[1]	7	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		6		22,822
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	603	[2]
Total, Fair Value		1,491		24,274
Total, Unrealized Losses		6	[1]	610	[2]
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		50		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[1]	0	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0		4,177
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	66	[2]
Total, Fair Value		50		4,177
Total, Unrealized Losses		0	[1]	66	[2]
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		20		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[1]	0	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0		63
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	2	[2]
Total, Fair Value		20		63
Total, Unrealized Losses		0	[1]	2	[2]
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		119		49
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1	[1]	1	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0		430
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	20	[2]
Total, Fair Value		119		479
Total, Unrealized Losses		1	[1]	21	[2]
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		0		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[1]	0	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		6		9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	0	[2]
Total, Fair Value		6		9
Total, Unrealized Losses		0	[1]	0	[2]
Temporarily Impaired Securities [Member] | Commercial Mortgage Backed 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		207		68
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		1	[1]	0	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0		1,986
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	69	[2]
Total, Fair Value		207		2,054
Total, Unrealized Losses		1	[1]	69	[2]
Temporarily Impaired Securities [Member] | Commercial Mortgage Backed 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				106
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	[2]			1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value				773
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[2]			26
Total, Fair Value				879
Total, Unrealized Losses	[2]			27
Temporarily Impaired Securities [Member] | Residential Mortgage Backed 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		1,089		1,229
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		4	[1]	5	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0		15,384
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1]	420	[2]
Total, Fair Value		1,089		16,613
Total, Unrealized Losses		$ 4	[1]	$ 425	[2]

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

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

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

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