Stockholders' Equity (Changes in Accumulated Other Comprehensive Income (Loss) by Component) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Adoption of ASUs 2016-01 and 2018-02			$ (82.4)
Net current period AOCI	$ 118.9	$ 53.1
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(9.1)	(20.9)
AOCI activity before reclassifications	(0.9)	5.7
Net current period AOCI	(0.9)	5.7
Ending balance	(10.0)	(15.2)
Changes in benefit plans net (loss) gains and prior service (cost)/credit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(52.6)	(70.2)
AOCI activity before reclassifications	2.6	2.2
Amounts reclassified from AOCI	0.2
Net current period AOCI	2.8	2.2
Ending balance	(49.8)	(68.0)
Unrealized net gains (losses) on AFS securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	9.6	(87.2)
AOCI activity before reclassifications	126.3	46.0
Amounts reclassified from AOCI	(9.3)	(0.8)
Net current period AOCI	117.0	45.2
Ending balance	126.6	(42.0)
Total AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(52.1)	(178.3)
AOCI activity before reclassifications	128.0	53.9
Amounts reclassified from AOCI	(9.1)	(0.8)
Net current period AOCI	118.9	53.1
Ending balance	$ 66.8	$ (125.2)