Accumulated Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 720.7	$ 735.0	$ 732.4	$ 718.8	$ 721.1	$ 733.9	$ 732.4	$ 733.9
Other comprehensive income, net of tax	10.8	15.9	9.1	8.2	11.9	(15.5)	35.8	4.6
Ending balance	721.0	720.7	735.0	719.3	718.8	721.1	721.0	719.3
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(18.4)		(19.8)	(31.2)		(33.2)	(19.8)	(33.2)
Less: income tax benefit (expense)	(0.2)			(0.3)			(0.7)	(0.9)
Net loss (gain) reclassified from AOCI to Net (loss) income	0.8			0.9			2.2	2.8
Translation impact on Canadian pension plan AOCI balance				(0.1)
Other comprehensive income, net of tax	0.8			0.8			2.2	2.8
Ending balance	(17.6)	(18.4)		(30.4)	(31.2)		(17.6)	(30.4)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI	0.1						0.3	0.2
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI	0.9			1.2			2.6	3.5
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				(0.2)		0.3		0.3
Unrealized gain (loss) on securities								1.4
Less: income tax benefit (expense)								(0.3)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges								1.1
Reclassification from AOCI								(2.1)
Less: income tax benefit (expense)								0.5
Net loss (gain) reclassified from AOCI to Net (loss) income								(1.6)
Other comprehensive income, net of tax								(0.5)
Ending balance				(0.2)	(0.2)			(0.2)
AOCI, Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	24.8		1.1	(10.7)		(5.6)	1.1	(5.6)
Unrealized gain (loss) on securities	29.8			7.8			72.1	(16.0)
Less: income tax benefit (expense)	(7.0)			(1.9)			(17.0)	4.0
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges	22.8			5.9			55.1	(12.0)
Reclassification from AOCI	(16.7)			1.8			(28.1)	18.1
Reclassification due to forecasted transactions no longer probable of occurring				0.1				0.8
Less: income tax benefit (expense)	3.9			(0.5)			6.7	(4.7)
Net loss (gain) reclassified from AOCI to Net (loss) income	(12.8)			1.4			(21.4)	14.2
Other comprehensive income, net of tax	10.0			7.3			33.7	2.2
Ending balance	34.8	24.8		(3.4)	(10.7)		34.8	(3.4)
AOCI, Excluded Component of Derivative Qualifying as Fair Value Hedge Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance						0.0		0.0
Unrealized gain (loss) on securities								(1.0)
Less: income tax benefit (expense)								0.2
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges								(0.8)
Reclassification from AOCI								0.4
Reclassification due to cancellation of firm commitment								0.6
Less: income tax benefit (expense)								(0.2)
Net loss (gain) reclassified from AOCI to Net (loss) income								0.8
Ending balance				0.0				0.0
AOCI, Accumulated Foreign Currency Adjustment Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.1)			(0.1)		(0.1)		(0.1)
Other comprehensive income, net of tax				0.1			(0.1)	0.1
Ending balance	(0.1)	(0.1)			(0.1)		(0.1)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	6.3	(9.6)	(18.7)	(42.2)	(54.1)	(38.6)	(18.7)	(38.6)
Other comprehensive income, net of tax	10.8	15.9	9.1	8.2	11.9	(15.5)
Ending balance	$ 17.1	$ 6.3	$ (9.6)	$ (34.0)	$ (42.2)	$ (54.1)	$ 17.1	$ (34.0)