Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Amortized Cost	$ 34,456.1	$ 33,864.8
Gross unrealized, gains	71.8	66.6
Gross unrealized, losses	269.9	189.3
Fair Value	34,258.0	33,742.1
U.S. Government
Investment Holdings [Line Items]
Amortized Cost	5,678.2	5,714.4
Gross unrealized, gains	25.6	18.0
Gross unrealized, losses	52.1	32.1
Fair Value	5,651.7	5,700.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Amortized Cost	690.3	749.9
Gross unrealized, gains	0.5	0.0
Gross unrealized, losses	3.1	3.5
Fair Value	687.7	746.4
Government Sponsored Agency
Investment Holdings [Line Items]
Amortized Cost	19,668.2	18,745.3
Gross unrealized, gains	34.4	39.9
Gross unrealized, losses	147.9	108.6
Fair Value	19,554.7	18,676.6
Non-U.S. Government
Investment Holdings [Line Items]
Amortized Cost	143.2	179.1
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	2.2	1.9
Fair Value	141.0	177.2
Corporate Debt
Investment Holdings [Line Items]
Amortized Cost	2,732.0	3,013.7
Gross unrealized, gains	3.6	2.2
Gross unrealized, losses	28.9	22.9
Fair Value	2,706.7	2,993.0
Covered Bonds
Investment Holdings [Line Items]
Amortized Cost	848.6	879.0
Gross unrealized, gains	0.8	1.0
Gross unrealized, losses	7.2	4.4
Fair Value	842.2	875.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Amortized Cost	1,983.2	1,819.8
Gross unrealized, gains	5.3	4.0
Gross unrealized, losses	5.5	3.8
Fair Value	1,983.0	1,820.0
Other Asset-Backed
Investment Holdings [Line Items]
Amortized Cost	2,239.7	2,297.7
Gross unrealized, gains	1.3	1.5
Gross unrealized, losses	16.8	7.9
Fair Value	2,224.2	2,291.3
Auction Rate
Investment Holdings [Line Items]
Amortized Cost	3.0	4.4
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.1	0.1
Fair Value	2.9	4.3
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Amortized Cost	448.7	439.2
Gross unrealized, gains	0.3	0.0
Gross unrealized, losses	6.1	4.1
Fair Value	442.9	435.1
Other
Investment Holdings [Line Items]
Amortized Cost	21.0	22.3
Gross unrealized, gains	0.0	0.0
Gross unrealized, losses	0.0	0.0
Fair Value	$ 21.0	$ 22.3

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
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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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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