Securities Available for Sale (Addition Information) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains		$ 4.0		$ 24.5		$ 28.0
Available-for-sale Securities, Gross Realized Losses		0.0		3.0		$ 42.0
Available-for-sale Securities		30,672.0		27,825.0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	[1]	642.0		533.0
Federal Home Loan Bank (FHLB) of Atlanta stock (par value)		132.0		32.0
Federal Reserve Bank Stock		402.0		402.0
Mutual fund investments (par value)		102.0		93.0
Fair Value, Inputs, Level 3 [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	[2]	633.0		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		633.0		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		540.0	[3]	440.0	[4]
Investments, Fair Value Disclosure		$ 6.0		$ 6.0

[1] 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.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 $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.

[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.