Stockholders' Equity (Before- and After-Tax components of OCI) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	$ 160.4	$ 66.1
Tax	(41.5)	(13.0)
Net current period AOCI	118.9	53.1
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	(0.6)	2.3
Tax	(0.3)	3.4
Net current period AOCI	(0.9)	5.7
Foreign currency translation adjustments | AOCI activity before reclassification
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	(0.6)	2.3
Tax	(0.3)	3.4
Net current period AOCI	(0.9)	5.7
Changes in benefit plan net loss and prior service (cost)/credit losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	3.8	2.9
Tax	(1.0)	(0.7)
Net current period AOCI	2.8	2.2
Changes in benefit plan net loss and prior service (cost)/credit losses | Reclassifications Out of AOCI | Operating Expenses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	0.2
Net current period AOCI	0.2
Changes in benefit plan net loss and prior service (cost)/credit losses | AOCI activity before reclassification
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	3.6	2.9
Tax	(1.0)	(0.7)
Net current period AOCI	2.6	2.2
Unrealized net gains on securities AFS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	157.2	60.9
Tax	(40.2)	(15.7)
Net current period AOCI	117.0	45.2
Unrealized net gains on securities AFS | Reclassifications Out of AOCI | Other Non-interest income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	(12.5)	(1.1)
Tax	3.2	0.3
Net current period AOCI	(9.3)	(0.8)
Unrealized net gains on securities AFS | AOCI activity before reclassification
Accumulated Other Comprehensive Income (Loss) [Line Items]
Gross Amount	169.7	62.0
Tax	(43.4)	(16.0)
Net current period AOCI	$ 126.3	$ 46.0