Accumulated Other Comprehensive (Loss) Income - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 733.9	$ 740.4	$ 746.3
Other comprehensive income (loss), net of tax	19.9	10.2	(11.7)
Ending balance	732.4	733.9	740.4
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(33.2)	(35.6)	(38.5)
Less: income tax (expense) benefit	(1.2)	(1.4)	(1.5)
Net loss (gain) reclassified from AOCI to Net income	3.9	4.7	4.8
Translation impact on Canadian pension plan AOCI balance	(0.1)	(0.1)
Other comprehensive income (loss), net of tax	13.4	2.4	2.9
Ending balance	(19.8)	(33.2)	(35.6)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Unrealized gain (loss) on securities	0.4	(0.4)	4.4
Less: income tax (expense) benefit	(0.1)	0.1	(1.1)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges	0.3	(0.3)	3.3
Reclassification from AOCI	0.4	0.5	0.9
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Unrealized gain (loss) on securities	12.3	(2.5)	(6.9)
Less: income tax (expense) benefit	(3.0)	0.6	1.7
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges	9.3	(1.9)	(5.2)
Reclassification from AOCI	4.7	5.6	5.4
AOCI, Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5.6)	(13.4)	0.5
Unrealized gain (loss) on securities	(9.4)	(9.5)	(21.2)
Less: income tax (expense) benefit	2.3	2.3	5.3
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges	(7.1)	(7.2)	(15.9)
Reclassification from AOCI	17.4	19.7	2.7
Reclassification due to forecasted transactions no longer probable of occurring	0.8
Less: income tax (expense) benefit	(4.4)	(4.7)	(0.7)
Net loss (gain) reclassified from AOCI to Net income	13.8	15.0	2.0
Other comprehensive income (loss), net of tax	6.7	7.8	(13.9)
Ending balance	1.1	(5.6)	(13.4)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.3	0.3	0.9
Unrealized gain (loss) on securities	1.3	4.4	4.7
Less: income tax (expense) benefit	(0.7)	(1.1)	(1.1)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges	0.6	3.3	3.6
Reclassification from AOCI	(2.0)	(4.4)	(5.4)
Less: income tax (expense) benefit	1.1	1.1	1.2
Net loss (gain) reclassified from AOCI to Net income	(0.9)	(3.3)	(4.2)
Other comprehensive income (loss), net of tax	(0.3)		(0.6)
Ending balance		0.3	0.3
AOCI, Excluded Component of Derivative Qualifying as Fair Value Hedge Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Unrealized gain (loss) on securities	(1.0)
Less: income tax (expense) benefit	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 (expense) benefit	(0.2)
Net loss (gain) reclassified from AOCI to Net income	0.8
AOCI, Accumulated Foreign Currency Adjustment Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.1)	(0.1)
Other comprehensive income (loss), net of tax	0.1		(0.1)
Ending balance		(0.1)	(0.1)
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(38.6)	(48.8)	(36.7)
Other comprehensive income (loss), net of tax	19.9	10.2	(11.7)
Ending balance	$ (18.7)	$ (38.6)	$ (48.8)