Securities - Reconciliation of Amortized Cost to Fair Values of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	$ 13,049.0	$ 8,921.1
GROSS UNREALIZED GAINS	22.0	29.2
GROSS UNREALIZED LOSSES	60.1	45.2
FAIR VALUE	13,010.9	8,905.1
U.S. Government Obligations
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	35.0	15.0
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	35.0	15.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	34.6	63.6
GROSS UNREALIZED GAINS	1.4	2.7
GROSS UNREALIZED LOSSES	0.1	0.0
FAIR VALUE	35.9	66.3
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	5.8	7.4
GROSS UNREALIZED GAINS	0.4	0.5
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	6.2	7.9
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	431.5	231.2
GROSS UNREALIZED GAINS	1.0	0.2
GROSS UNREALIZED LOSSES	0.4	0.4
FAIR VALUE	432.1	231.0
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2,821.5	2,051.6
GROSS UNREALIZED GAINS	11.9	10.1
GROSS UNREALIZED LOSSES	3.7	3.7
FAIR VALUE	2,829.7	2,058.0
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	5,536.2	3,517.5
GROSS UNREALIZED GAINS	1.3	4.9
GROSS UNREALIZED LOSSES	6.0	2.3
FAIR VALUE	5,531.5	3,520.1
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	43.8	606.0
GROSS UNREALIZED GAINS	0.0	0.2
GROSS UNREALIZED LOSSES	0.1	0.1
FAIR VALUE	43.7	606.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	2,788.9	2,154.7
GROSS UNREALIZED GAINS	5.4	10.5
GROSS UNREALIZED LOSSES	4.1	2.8
FAIR VALUE	2,790.2	2,162.4
Other Asset-Backed
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	1,175.8	143.4
GROSS UNREALIZED GAINS	0.6	0.1
GROSS UNREALIZED LOSSES	0.5	0.0
FAIR VALUE	1,175.9	143.5
Other
Schedule of Held-to-maturity Securities [Line Items]
AMORTIZED COST	175.9	130.7
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	45.2	35.9
FAIR VALUE	$ 130.7	$ 94.8