Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Stockholders' Equity Attributable to Parent	$ 4,151.2		$ 4,151.2			$ 4,563.4
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax					$ (44.9)	(45.7)		$ (100.6)
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax					(2.1)	(2.3)		(2.7)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Stockholders' Equity Attributable to Parent	(3.6)	$ (1.9)	(3.6)	$ (1.9)	(4.0)	(2.1)	$ (2.9)	0.0
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.4	1.0	(1.5)	(1.9)
Reclassification from AOCI, Current Period, Tax	0.0	0.0	0.0	0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.1)	(0.3)	0.4	0.5
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.4	1.0	(1.5)	(1.9)
Accumulated Other Comprehensive Income
Stockholders' Equity Attributable to Parent	479.4	549.0	479.4	549.0	543.0	680.5	497.3	336.1
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	(56.1)	58.8	(174.3)	225.9
Reclassification from AOCI, Current Period, Tax	2.0	1.9	7.8	3.4
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(7.5)	(7.1)	(26.8)	(13.0)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	16.7	(13.8)	54.2	(56.6)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(63.6)	51.7	(201.1)	212.9
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	(45.0)	(98.8)	(45.0)	(98.8)			99.5
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0	0.7	0.0	1.8
Reclassification from AOCI, Current Period, Tax	0.1	0.0	0.5	0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(0.1)	0.0	0.7	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.1	(0.2)	0.5	(0.5)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.1)	0.7	0.7	1.8
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Stockholders' Equity Attributable to Parent	(2.0)	(2.5)	(2.0)	(2.5)			(2.6)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0	0.1	0.0	0.2
Reclassification from AOCI, Current Period, Tax	(0.1)	0.0	(0.1)	0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.1	0.0	0.3	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.1)	(0.1)	(0.1)	(0.1)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.1	0.1	0.3	0.2
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Stockholders' Equity Attributable to Parent	530.0	652.2	530.0	652.2	$ 594.0	$ 730.6	$ 602.3	$ 439.4
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	(56.5)	57.0	(172.8)	225.8
Reclassification from AOCI, Current Period, Tax	2.0	1.9	7.4	3.4
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(7.5)	(7.1)	(27.8)	(13.0)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	16.8	(13.2)	53.4	(56.5)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ (64.0)	$ 49.9	$ (200.6)	$ 212.8