Securities - Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
AMORTIZED COST	$ 37,041.8	$ 33,864.8
GROSS UNREALIZED GAINS	98.4	66.6
GROSS UNREALIZED LOSSES	251.4	189.3
FAIR VALUE	36,888.8	33,742.1
U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	5,203.1	5,714.4
GROSS UNREALIZED GAINS	21.8	18.0
GROSS UNREALIZED LOSSES	39.6	32.1
FAIR VALUE	5,185.3	5,700.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
AMORTIZED COST	657.6	749.9
GROSS UNREALIZED GAINS	2.0	0.0
GROSS UNREALIZED LOSSES	3.7	3.5
FAIR VALUE	655.9	746.4
Government Sponsored Agency
Investment Holdings [Line Items]
AMORTIZED COST	22,522.7	18,745.3
GROSS UNREALIZED GAINS	52.4	39.9
GROSS UNREALIZED LOSSES	150.5	108.6
FAIR VALUE	22,424.6	18,676.6
Non-U.S. Government
Investment Holdings [Line Items]
AMORTIZED COST	143.3	179.1
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	1.1	1.9
FAIR VALUE	142.2	177.2
Corporate Debt
Investment Holdings [Line Items]
AMORTIZED COST	2,312.6	3,013.7
GROSS UNREALIZED GAINS	3.2	2.2
GROSS UNREALIZED LOSSES	21.1	22.9
FAIR VALUE	2,294.7	2,993.0
Covered Bonds
Investment Holdings [Line Items]
AMORTIZED COST	832.7	879.0
GROSS UNREALIZED GAINS	1.4	1.0
GROSS UNREALIZED LOSSES	4.8	4.4
FAIR VALUE	829.3	875.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
AMORTIZED COST	2,087.8	1,819.8
GROSS UNREALIZED GAINS	11.9	4.0
GROSS UNREALIZED LOSSES	3.5	3.8
FAIR VALUE	2,096.2	1,820.0
Other Asset-Backed
Investment Holdings [Line Items]
AMORTIZED COST	2,678.9	2,297.7
GROSS UNREALIZED GAINS	1.7	1.5
GROSS UNREALIZED LOSSES	22.9	7.9
FAIR VALUE	2,657.7	2,291.3
Auction Rate
Investment Holdings [Line Items]
AMORTIZED COST		4.4
GROSS UNREALIZED GAINS		0.0
GROSS UNREALIZED LOSSES		0.1
FAIR VALUE		4.3
Commercial Mortgage-Backed
Investment Holdings [Line Items]
AMORTIZED COST	587.4	439.2
GROSS UNREALIZED GAINS	4.0	0.0
GROSS UNREALIZED LOSSES	4.2	4.1
FAIR VALUE	587.2	435.1
Other
Investment Holdings [Line Items]
AMORTIZED COST	15.7	22.3
GROSS UNREALIZED GAINS	0.0	0.0
GROSS UNREALIZED LOSSES	0.0	0.0
FAIR VALUE	$ 15.7	$ 22.3

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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Amount, before tax, of unrealized loss 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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