Securities (Reconciliation of Amortized Cost to Fair Value of Securities Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Total	$ 5,869.6	$ 5,248.3
Gross Unrealized Gains	28.1	19.5
Gross Unrealized Losses	36.2	40.3
Fair Value	5,861.5	5,227.5
U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	28.0	26.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	28.0	26.0
Obligations of States and Political Subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Total	79.3	89.2
Gross Unrealized Gains	4.9	5.2
Gross Unrealized Losses	0.0	0.0
Fair Value	84.2	94.4
Government Sponsored Agency
Schedule of Held-to-maturity Securities [Line Items]
Total	9.2	9.9
Gross Unrealized Gains	0.7	0.7
Gross Unrealized Losses	0.0	0.0
Fair Value	9.9	10.6
Corporate Debt
Schedule of Held-to-maturity Securities [Line Items]
Total	85.5
Gross Unrealized Gains	0.1
Gross Unrealized Losses	0.0
Fair Value	85.6
Covered Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	1,578.4	892.4
Gross Unrealized Gains	5.2	0.4
Gross Unrealized Losses	1.6	1.9
Fair Value	1,582.0	890.9
Non-U.S. Government
Schedule of Held-to-maturity Securities [Line Items]
Total	1,077.2	1,118.0
Gross Unrealized Gains	6.7	4.8
Gross Unrealized Losses	0.0	0.5
Fair Value	1,083.9	1,122.3
Certificates of Deposit
Schedule of Held-to-maturity Securities [Line Items]
Total	730.5	691.6
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.2	0.1
Fair Value	730.4	691.6
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	2,186.8	2,326.2
Gross Unrealized Gains	10.4	8.3
Gross Unrealized Losses	0.5	0.9
Fair Value	2,196.7	2,333.6
Other
Schedule of Held-to-maturity Securities [Line Items]
Total	94.7	95.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	33.9	36.9
Fair Value	$ 60.8	$ 58.1