Securities Available for Sale (Addition Information) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017		Mar. 31, 2016	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 0.0		$ 0.0
Available-for-sale Securities, Gross Realized Losses		0.0		$ 0.0
Available-for-sale Securities	[1]	31,127.0			$ 30,672.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	[2]	638.0			642.0
Federal Home Loan Bank (FHLB) of Atlanta stock (par value)		111.0			132.0
Federal Reserve Bank Stock		402.0			402.0
Mutual fund investments (par value)		119.0			102.0
Fair Value, Inputs, Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	[3]	604.0			633.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		604.0			633.0
Equity Securities [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		515.0	[4]		540.0	[5]
Investments, Fair Value Disclosure		$ 6.0			$ 6.0

[1] Includes securities AFS pledged as collateral where counterparties have the right to sell or repledge the collateral of $100 million and $0 million at March 31, 2017 and December 31, 2016, respectively.

[2] At March 31, 2017, the fair value of other equity securities was comprised of the following: $111 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $119 million of mutual fund investments, and $6 million of other.At December 31, 2016, the fair value of other equity securities was comprised of the following: $132 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $102 million of mutual fund investments, and $6 million of other.

[3] Trading assets and derivative instruments, securities AFS, and trading liabilities and derivative instruments that are classified as level 1 are valued based on quoted market prices. For those instruments classified as level 2 or 3, refer to the respective valuation discussions within this footnote.

[4] Includes $119 million of mutual fund investments, $111 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, and $6 million of other.

[5] Includes $102 million of mutual fund investments, $132 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, and $6 million of other.