Securities - Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 12,284.5	$ 14,354.0
GROSS UNREALIZED GAINS	45.7	18.7
GROSS UNREALIZED LOSSES	80.9	105.7
FAIR VALUE	12,249.3	14,267.0
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	138.8	101.6
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	138.8	101.6
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	10.1	18.9
GROSS UNREALIZED GAINS	0.2	0.6
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	10.3	19.5
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	4.1	4.5
GROSS UNREALIZED GAINS	0.2	0.2
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	4.3	4.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	4,076.0	6,488.2
GROSS UNREALIZED GAINS	5.3	2.1
GROSS UNREALIZED LOSSES	2.5	8.7
FAIR VALUE	4,078.8	6,481.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	405.1	472.9
GROSS UNREALIZED GAINS	1.4	0.4
GROSS UNREALIZED LOSSES	0.3	1.8
FAIR VALUE	406.2	471.5
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,006.7	2,877.6
GROSS UNREALIZED GAINS	16.1	9.6
GROSS UNREALIZED LOSSES	2.4	9.3
FAIR VALUE	3,020.4	2,877.9
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	262.9	45.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	262.9	45.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	3,285.4	2,966.8
GROSS UNREALIZED GAINS	21.7	5.8
GROSS UNREALIZED LOSSES	2.1	12.3
FAIR VALUE	3,305.0	2,960.3
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	804.3	1,146.4
GROSS UNREALIZED GAINS	0.7	0.0
GROSS UNREALIZED LOSSES	0.3	4.0
FAIR VALUE	804.7	1,142.4
Other
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	291.1	232.0
GROSS UNREALIZED GAINS	0.1	0.0
GROSS UNREALIZED LOSSES	73.3	69.6
FAIR VALUE	$ 217.9	$ 162.4

Amount of investment in debt security measured at amortized cost (held-to-maturity).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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