Securities with Unrealized Losses (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		$ 11,409	[1]	$ 19,459	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		63	[3]	383	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		6,738	[1]	464	[2]
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		156	[3]	9	[4]
Total, Fair Value		18,147	[1]	19,923	[2]
Total, Unrealized Losses		219	[3]	392	[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		0	[1]	16	[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]	17	[2]
Total, Unrealized Losses		0	[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] | Residential 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	[2]			16
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	[4]			0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	[2]			0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	[4]			0
Total, Fair Value	[2]			16
Total, Unrealized Losses	[4]			0
Temporarily Impaired Securities [Member]
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		11,409		19,443
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		63	[3]	383	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		6,737		463
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		156	[3]	9	[4]
Total, Fair Value		18,146		19,906
Total, Unrealized Losses		219	[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		1,993		4,380
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		12	[3]	86	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		841		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		20	[3]	0	[4]
Total, Fair Value		2,834		4,380
Total, Unrealized Losses		32	[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		23		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		60		3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		1	[3]	0	[4]
Total, Fair Value		83		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		267		149
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		3	[3]	5	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		114		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		5	[3]	0	[4]
Total, Fair Value		381		149
Total, Unrealized Losses		8	[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		4		5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		0	[3]	0	[4]
Total, Fair Value		4		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		2		4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		3	[3]	1	[4]
Total, Fair Value		2		4
Total, Unrealized Losses		3	[3]	1	[4]
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		887		1,117
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		9	[3]	38	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		915		15
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		29	[3]	1	[4]
Total, Fair Value		1,802		1,132
Total, Unrealized Losses		38	[3]	39	[4]
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		134		184
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		93		0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		2	[3]	0	[4]
Total, Fair Value		227		184
Total, Unrealized Losses		3	[3]	5	[4]
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		8,095		13,505
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		38	[3]	247	[4]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		4,708		436
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss		96	[3]	7	[4]
Total, Fair Value		12,803		13,941
Total, Unrealized Losses		$ 134	[3]	$ 254	[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.