Securities - Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 14,354.0	$ 13,049.0
GROSS UNREALIZED GAINS	18.7	22.0
GROSS UNREALIZED LOSSES	105.7	60.1
FAIR VALUE	14,267.0	13,010.9
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	101.6	35.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	101.6	35.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	18.9	34.6
GROSS UNREALIZED GAINS	0.6	1.4
GROSS UNREALIZED LOSSES	0.0	0.1
FAIR VALUE	19.5	35.9
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	4.5	5.8
GROSS UNREALIZED GAINS	0.2	0.4
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	4.7	6.2
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	472.9	431.5
GROSS UNREALIZED GAINS	0.4	1.0
GROSS UNREALIZED LOSSES	1.8	0.4
FAIR VALUE	471.5	432.1
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2,877.6	2,821.5
GROSS UNREALIZED GAINS	9.6	11.9
GROSS UNREALIZED LOSSES	9.3	3.7
FAIR VALUE	2,877.9	2,829.7
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	6,488.2	5,536.2
GROSS UNREALIZED GAINS	2.1	1.3
GROSS UNREALIZED LOSSES	8.7	6.0
FAIR VALUE	6,481.6	5,531.5
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	45.1	43.8
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.1
FAIR VALUE	45.1	43.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2,966.8	2,788.9
GROSS UNREALIZED GAINS	5.8	5.4
GROSS UNREALIZED LOSSES	12.3	4.1
FAIR VALUE	2,960.3	2,790.2
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,146.4	1,175.8
GROSS UNREALIZED GAINS	0.0	0.6
GROSS UNREALIZED LOSSES	4.0	0.5
FAIR VALUE	1,142.4	1,175.9
Other
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	232.0	175.9
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	69.6	45.2
FAIR VALUE	$ 162.4	$ 130.7