Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021	Dec. 31, 2020
Stockholders' Equity Attributable to Parent	$ 3,394.5		$ 4,007.7
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax			(52.1)	$ (45.7)
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax			(1.9)	(2.3)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Stockholders' Equity Attributable to Parent	(9.5)	$ (3.8)	(3.7)	(2.1)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(5.8)	(1.7)
Reclassification from AOCI, Current Period, Tax		0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	1.6	0.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(5.8)	(1.7)
Accumulated Other Comprehensive Income
Stockholders' Equity Attributable to Parent	(62.3)	390.5	448.1	680.5
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(516.2)	(282.3)
Reclassification from AOCI, Current Period, Tax	(1.5)	2.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	5.8	(7.7)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	135.6	75.7
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(510.4)	(290.0)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	(52.1)	(44.9)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	0.8
Reclassification from AOCI, Current Period, Tax	0.0	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.0	0.5
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0	0.8
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Stockholders' Equity Attributable to Parent	2.8	(2.2)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	4.7	0.1
Reclassification from AOCI, Current Period, Tax	0.0	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(1.2)	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	4.7	0.1
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Stockholders' Equity Attributable to Parent	(3.5)	441.4	$ 505.8	$ 730.6
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(515.1)	(281.5)
Reclassification from AOCI, Current Period, Tax	(1.5)	2.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	5.8	(7.7)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	135.2	74.8
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ (509.3)	$ (289.2)