Securities - Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
AMORTIZED COST	$ 38,722.2	$ 37,041.8
GROSS UNREALIZED GAINS	262.7	98.4
GROSS UNREALIZED LOSSES	108.6	251.4
FAIR VALUE	38,876.3	36,888.8
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	4,527.5	5,203.1
GROSS UNREALIZED GAINS	26.7	21.8
GROSS UNREALIZED LOSSES	5.1	39.6
FAIR VALUE	4,549.1	5,185.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	1,604.0	657.6
GROSS UNREALIZED GAINS	24.6	2.0
GROSS UNREALIZED LOSSES	13.3	3.7
FAIR VALUE	1,615.3	655.9
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	23,247.5	22,522.7
GROSS UNREALIZED GAINS	101.8	52.4
GROSS UNREALIZED LOSSES	78.1	150.5
FAIR VALUE	23,271.2	22,424.6
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	3.3	143.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	1.1
FAIR VALUE	3.3	142.2
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	2,378.9	2,312.6
GROSS UNREALIZED GAINS	27.8	3.2
GROSS UNREALIZED LOSSES	4.0	21.1
FAIR VALUE	2,402.7	2,294.7
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	766.3	832.7
GROSS UNREALIZED GAINS	4.4	1.4
GROSS UNREALIZED LOSSES	0.8	4.8
FAIR VALUE	769.9	829.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,091.3	2,087.8
GROSS UNREALIZED GAINS	37.4	11.9
GROSS UNREALIZED LOSSES	1.1	3.5
FAIR VALUE	2,127.6	2,096.2
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	3,324.5	2,678.9
GROSS UNREALIZED GAINS	11.3	1.7
GROSS UNREALIZED LOSSES	5.3	22.9
FAIR VALUE	3,330.5	2,657.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	769.9	587.4
GROSS UNREALIZED GAINS	28.7	4.0
GROSS UNREALIZED LOSSES	0.9	4.2
FAIR VALUE	797.7	587.2
Other
Investment Holdings [Line Items]
AMORTIZED COST	9.0	15.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	$ 9.0	$ 15.7

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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