Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale (Detail) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Amortized Cost	$ 32,367.8	$ 29,514.1
Gross Unrealized Gains	50.3	110.9
Gross Unrealized Losses	100.2	66.5
Fair Value	32,317.9	29,558.5
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	6,180.4	4,493.5
Gross Unrealized Gains	3.4	15.1
Gross Unrealized Losses	5.5	1.7
Fair Value	6,178.3	4,506.9
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	36.4	4.5
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.1	0.0
Fair Value	36.4	4.6
U.S. Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	16,370.5	16,326.4
Gross Unrealized Gains	42.8	82.3
Gross Unrealized Losses	46.5	19.5
Fair Value	16,366.8	16,389.2
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	309.5	309.5
Gross Unrealized Gains	0.1	0.9
Gross Unrealized Losses	0.1	0.0
Fair Value	309.5	310.4
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	3,744.4	3,617.5
Gross Unrealized Gains	0.9	1.8
Gross Unrealized Losses	33.1	41.6
Fair Value	3,712.2	3,577.7
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	1,873.3	1,899.9
Gross Unrealized Gains	1.8	7.9
Gross Unrealized Losses	4.9	0.3
Fair Value	1,870.2	1,907.5
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	860.9
Gross Unrealized Gains	0.5
Gross Unrealized Losses	2.0
Fair Value	859.4
Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost		360.0
Gross Unrealized Gains		1.5
Gross Unrealized Losses		0.9
Fair Value		360.6
Residential Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost		6.9
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.5
Fair Value		6.4
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,504.2	2,321.8
Gross Unrealized Gains	0.1	0.5
Gross Unrealized Losses	4.2	1.0
Fair Value	2,500.1	2,321.3
Auction Rate Securities
Investment Holdings [Line Items]
Amortized Cost	16.7	18.4
Gross Unrealized Gains	0.5	0.5
Gross Unrealized Losses	0.1	0.8
Fair Value	17.1	18.1
Commercial Mortgage Backed Securities
Investment Holdings [Line Items]
Amortized Cost	378.1
Gross Unrealized Gains	0.0
Gross Unrealized Losses	3.7
Fair Value	374.4
Others
Investment Holdings [Line Items]
Amortized Cost	93.4	155.7
Gross Unrealized Gains	0.1	0.3
Gross Unrealized Losses	0.0	0.2
Fair Value	$ 93.5	$ 155.8

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Topic 320
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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