Securities - Remaining Maturity of Securities Held to Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Held to Maturity - Amortized Cost
U.S. Government	$ 4,554.3
Obligations of States and Political Subdivisions	7,309.7
Corporate Debt	317.3
Covered Bonds	103.2
AMORTIZED COST	12,284.5	$ 14,354.0
Held to Maturity - Fair Value
U.S. Government	4,556.4
Obligations of States and Political Subdivisions	7,333.1
Corporate Debt	310.7
Covered Bonds	49.1
FAIR VALUE	12,249.3	14,267.0
U.S. Government
Held to Maturity - Amortized Cost
U.S. Government	138.8
Obligations of States and Political Subdivisions	0.0
Corporate Debt	0.0
Covered Bonds	0.0
AMORTIZED COST	138.8	101.6
Held to Maturity - Fair Value
U.S. Government	138.8
Obligations of States and Political Subdivisions	0.0
Corporate Debt	0.0
Covered Bonds	0.0
FAIR VALUE	138.8	101.6
Obligations of States and Political Subdivisions
Held to Maturity - Amortized Cost
U.S. Government	8.1
Obligations of States and Political Subdivisions	2.0
Corporate Debt	0.0
Covered Bonds	0.0
AMORTIZED COST	10.1	18.9
Held to Maturity - Fair Value
U.S. Government	8.2
Obligations of States and Political Subdivisions	2.1
Corporate Debt	0.0
Covered Bonds	0.0
FAIR VALUE	10.3	19.5
Government Sponsored Agency
Held to Maturity - Amortized Cost
U.S. Government	0.6
Obligations of States and Political Subdivisions	1.7
Corporate Debt	1.2
Covered Bonds	0.6
AMORTIZED COST	4.1	4.5
Held to Maturity - Fair Value
U.S. Government	0.6
Obligations of States and Political Subdivisions	1.8
Corporate Debt	1.2
Covered Bonds	0.7
FAIR VALUE	4.3	4.7
Non-U.S. Government
Held to Maturity - Amortized Cost
U.S. Government	2,757.9
Obligations of States and Political Subdivisions	1,318.1
Corporate Debt	0.0
Covered Bonds	0.0
AMORTIZED COST	4,076.0	6,488.2
Held to Maturity - Fair Value
U.S. Government	2,757.8
Obligations of States and Political Subdivisions	1,321.0
Corporate Debt	0.0
Covered Bonds	0.0
FAIR VALUE	4,078.8	6,481.6
Corporate Debt
Held to Maturity - Amortized Cost
U.S. Government	45.9
Obligations of States and Political Subdivisions	359.2
Corporate Debt	0.0
Covered Bonds	0.0
AMORTIZED COST	405.1	472.9
Held to Maturity - Fair Value
U.S. Government	46.2
Obligations of States and Political Subdivisions	360.0
Corporate Debt	0.0
Covered Bonds	0.0
FAIR VALUE	406.2	471.5
Covered Bonds
Held to Maturity - Amortized Cost
U.S. Government	599.8
Obligations of States and Political Subdivisions	2,406.9
Corporate Debt	0.0
Covered Bonds	0.0
AMORTIZED COST	3,006.7	2,877.6
Held to Maturity - Fair Value
U.S. Government	601.6
Obligations of States and Political Subdivisions	2,418.8
Corporate Debt	0.0
Covered Bonds	0.0
FAIR VALUE	3,020.4	2,877.9
Certificates of Deposit
Held to Maturity - Amortized Cost
U.S. Government	262.9
Obligations of States and Political Subdivisions	0.0
Corporate Debt	0.0
Covered Bonds	0.0
AMORTIZED COST	262.9	45.1
Held to Maturity - Fair Value
U.S. Government	262.9
Obligations of States and Political Subdivisions	0.0
Corporate Debt	0.0
Covered Bonds	0.0
FAIR VALUE	262.9	45.1
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Held to Maturity - Amortized Cost
U.S. Government	577.8
Obligations of States and Political Subdivisions	2,691.5
Corporate Debt	16.1
Covered Bonds	0.0
AMORTIZED COST	3,285.4	2,966.8
Held to Maturity - Fair Value
U.S. Government	577.8
Obligations of States and Political Subdivisions	2,711.4
Corporate Debt	15.8
Covered Bonds	0.0
FAIR VALUE	3,305.0	2,960.3
Other Asset-Backed
Held to Maturity - Amortized Cost
U.S. Government	151.9
Obligations of States and Political Subdivisions	398.3
Corporate Debt	254.1
Covered Bonds	0.0
AMORTIZED COST	804.3	1,146.4
Held to Maturity - Fair Value
U.S. Government	152.0
Obligations of States and Political Subdivisions	398.7
Corporate Debt	254.0
Covered Bonds	0.0
FAIR VALUE	804.7	1,142.4
Other
Held to Maturity - Amortized Cost
U.S. Government	10.6
Obligations of States and Political Subdivisions	132.0
Corporate Debt	45.9
Covered Bonds	102.6
AMORTIZED COST	291.1	232.0
Held to Maturity - Fair Value
U.S. Government	10.5
Obligations of States and Political Subdivisions	119.3
Corporate Debt	39.7
Covered Bonds	48.4
FAIR VALUE	$ 217.9	$ 162.4

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

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