Securities with Unrealized Losses (Detail) - USD ($) $ in Millions		Sep. 30, 2018		Dec. 31, 2017
Investments, Unrealized Loss Position [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		$ 14,937	[1]	$ 11,409	[2],[3]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		415	[1],[4]	63	[2],[3],[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		12,345	[1]	6,736	[2],[3]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		620	[1],[4]	153	[2],[3],[5]
Total, Fair Value		27,282	[1]	18,145	[2],[3]
Total, Unrealized Losses		1,035	[1],[4]	216	[2],[3],[5]
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]	0	[3]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[1],[4]	0	[3],[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0	[1]	1	[3]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[1],[4]	0	[3],[5]
Total, Fair Value		0	[1]	1	[3]
Total, Unrealized Losses		0	[1],[4]	0	[3],[5]
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	[3]			0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	[3],[5]			0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	[3]			1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[3],[5]			0
Total, Fair Value	[3]			1
Total, Unrealized Losses	[3],[5]			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		14,937		11,409	[2]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		415	[4]	63	[2],[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		12,345		6,735	[2]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		620	[4]	153	[2],[5]
Total, Fair Value		27,282		18,144	[2]
Total, Unrealized Losses		1,035	[4]	216	[2],[5]
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		2,554		1,993
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		77	[4]	12	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		1,579		841
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		65	[4]	20	[5]
Total, Fair Value		4,133		2,834
Total, Unrealized Losses		142	[4]	32	[5]
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		16		23
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[4]	0	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		62		60
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		3	[4]	1	[5]
Total, Fair Value		78		83
Total, Unrealized Losses		3	[4]	1	[5]
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		210		267
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		7	[4]	3	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		280		114
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		15	[4]	5	[5]
Total, Fair Value		490		381
Total, Unrealized Losses		22	[4]	8	[5]
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
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	[5]			0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value				4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[5]			0
Total, Fair Value				4
Total, Unrealized Losses	[5]			0
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		10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0	[4]	0	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		9		0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0	[4]	0	[5]
Total, Fair Value		9		10
Total, Unrealized Losses		0	[4]	0	[5]
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		1,029		887
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		25	[4]	9	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		1,519		915
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		87	[4]	29	[5]
Total, Fair Value		2,548		1,802
Total, Unrealized Losses		112	[4]	38	[5]
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		781		134
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		30	[4]	1	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		124		93
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		8	[4]	2	[5]
Total, Fair Value		905		227
Total, Unrealized Losses		38	[4]	3	[5]
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		10,347		8,095
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		276	[4]	38	[5]
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		8,772		4,708
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		442	[4]	96	[5]
Total, Fair Value		19,119		12,803
Total, Unrealized Losses		$ 718	[4]	$ 134	[5]

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

[2] Beginning January 1, 2018, the Company reclassified equity securities previously presented in Securities available for sale to Other assets on the Consolidated Balance Sheets. Reclassifications have been made to previously reported amounts for comparability.

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

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

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