Securities (Reconciliation of Amortized Cost to Fair Value of Securities Available for Sale) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Total	$ 37,095.7	$ 37,041.8
Gross Unrealized Gains	150.9	98.4
Gross Unrealized Losses	188.4	251.4
Fair Value	37,058.2	36,888.8
U.S. Government
Investment Holdings [Line Items]
Total	5,030.0	5,203.1
Gross Unrealized Gains	19.0	21.8
Gross Unrealized Losses	27.0	39.6
Fair Value	5,022.0	5,185.3
Obligations of States and Political Subdivisions
Investment Holdings [Line Items]
Total	755.8	657.6
Gross Unrealized Gains	10.7	2.0
Gross Unrealized Losses	1.7	3.7
Fair Value	764.8	655.9
Government Sponsored Agency
Investment Holdings [Line Items]
Total	22,624.9	22,522.7
Gross Unrealized Gains	68.3	52.4
Gross Unrealized Losses	124.6	150.5
Fair Value	22,568.6	22,424.6
Non-U.S. Government
Investment Holdings [Line Items]
Total	143.3	143.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.6	1.1
Fair Value	142.7	142.2
Corporate Debt
Investment Holdings [Line Items]
Total	2,297.6	2,312.6
Gross Unrealized Gains	10.2	3.2
Gross Unrealized Losses	14.1	21.1
Fair Value	2,293.7	2,294.7
Covered Bonds
Investment Holdings [Line Items]
Total	837.4	832.7
Gross Unrealized Gains	2.6	1.4
Gross Unrealized Losses	2.6	4.8
Fair Value	837.4	829.3
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Investment Holdings [Line Items]
Total	2,214.0	2,087.8
Gross Unrealized Gains	24.3	11.9
Gross Unrealized Losses	2.2	3.5
Fair Value	2,236.1	2,096.2
Other Asset-Backed
Investment Holdings [Line Items]
Total	2,530.5	2,678.9
Gross Unrealized Gains	3.3	1.7
Gross Unrealized Losses	13.6	22.9
Fair Value	2,520.2	2,657.7
Commercial Mortgage-Backed
Investment Holdings [Line Items]
Total	647.6	587.4
Gross Unrealized Gains	12.5	4.0
Gross Unrealized Losses	2.0	4.2
Fair Value	658.1	587.2
Other
Investment Holdings [Line Items]
Total	14.6	15.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 14.6	$ 15.7