Securities Available for Sale (Addition Information) (Detail) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2016		Sep. 30, 2015	Sep. 30, 2016		Sep. 30, 2015	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 0.0		$ 10.5	$ 4.0		$ 24.5
Available-for-sale Securities, Gross Realized Losses				$ 3.1			$ 3.1
Available-for-sale Securities		29,672.0			29,672.0			$ 27,825.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	[1]	655.0			655.0			533.0
Federal Home Loan Bank (FHLB) of Atlanta stock (par value)		143.0			143.0			32.0
Federal Reserve Bank Stock		402.0			402.0			402.0
Mutual fund investments (par value)		104.0			104.0			93.0
Fair Value, Inputs, Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	[2]	647.0			647.0			556.0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities		647.0			647.0			556.0
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		551.0	[3]		551.0	[3]		440.0	[4]
Investments, Fair Value Disclosure		$ 6.0			$ 6.0			$ 6.0

[1] At September 30, 2016, the fair value of other equity securities was comprised of the following: $143 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $104 million of mutual fund investments, and $6 million of other.At December 31, 2015, the fair value of other equity securities was comprised of the following: $32 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, $93 million of mutual fund investments, and $6 million of other.

[2] 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.

[3] Includes $104 million of mutual fund investments, $143 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, and $6 million of other.

[4] Includes $93 million of mutual fund investments, $32 million of FHLB of Atlanta stock, $402 million of Federal Reserve Bank of Atlanta stock, and $6 million of other.