Accumulated Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ 631.2		$ 692.5		$ 732.4
Other comprehensive income (loss), net of tax		9.9		6.9		15.0
Ending balance		652.2		631.2		692.5
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		2.8		(21.4)		(19.8)
Less: income tax (expense) benefit	[1]	(1.0)		(1.2)		(0.9)
Net loss (gain) reclassified from AOCI to Net income (loss)		3.2		3.7		3.0
Other comprehensive income (loss), net of tax		8.2		24.2		(1.6)
Ending balance		11.0		2.8		(21.4)
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		4.3		27.0		8.0
Less: income tax (expense) benefit		(1.0)		(6.5)		(1.9)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges		3.3		20.5		6.1
Reclassification from AOCI	[2]	(1.1)				0.4
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		2.2				(14.0)
Less: income tax (expense) benefit		(0.5)				3.3
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges		1.7				(10.7)
Reclassification from AOCI	[2]	5.3		4.9		3.5
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		0.4	[3]	17.7	[3]	1.1
Unrealized gain (loss) on securities		(10.7)		(12.2)		63.6
Less: income tax (expense) benefit		2.5		2.9		(15.0)
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges		(8.2)		(9.3)		48.6
Reclassification from AOCI		12.9		(10.5)		(41.9)
Less: income tax (expense) benefit		(3.0)		2.5		9.9
Net loss (gain) reclassified from AOCI to Net income (loss)		9.9		(8.0)		(32.0)
Other comprehensive income (loss), net of tax		1.7		(17.3)		16.6
Ending balance	[3]	2.1		0.4		17.7
Accumulated Other Comprehensive (Loss) Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		3.2		(3.7)		(18.7)
Other comprehensive income (loss), net of tax		9.9		6.9		15.0
Ending balance		$ 13.1		$ 3.2		$ (3.7)

[1] Income tax amounts reclassified out of AOCI were included as a component of Income tax (provision) benefit.

[2] Amounts amortized out of AOCI related to pension and other postretirement benefits were included within Net periodic postretirement benefit cost (see Note 5).

[3] As of December 31, 2023, we estimate a net mark-to-market gain before tax of $2.2 million in AOCI will be reclassified into Net income (loss) upon settlement within the next 12 months.