Pension and Postretirement Plans - Benefit Obligation and Funded Status (Details) - USD ($) $ in Millions		4 Months Ended	8 Months Ended	12 Months Ended
		Dec. 31, 2021	Sep. 07, 2021	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period				$ 393.8
Fair value of plan assets at end of period				400.9	$ 393.8
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period				439.5	611.9
Service cost		$ 0.0	$ 0.0	0.0	0.0
Interest cost		4.7	10.2	22.0	16.3
Hawaii Plan Adjustment				0.0	0.0
Plan Amendment				0.1	0.0
Actuarial loss (gain)	[1],[2]			8.8	(146.3)
Benefits paid				(36.7)	(34.9)
Settlements	[3]			0.0	(7.5)
Other				0.0	0.0
Benefit obligation at end of period		611.9		433.7	439.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period				390.6	559.2
Actual return (loss) on plan assets				41.7	(128.3)
Employer contributions				2.2	2.1
Benefits paid				(36.7)	(34.9)
Settlements	[3]			0.0	(7.5)
Fair value of plan assets at end of period		559.2		397.8	390.6
Unfunded status				(35.9)	(48.9)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period				102.4	100.9
Service cost		0.1	0.3	0.5	0.8
Interest cost		0.8	$ 1.8	5.2	4.2
Hawaii Plan Adjustment				0.0	45.9
Plan Amendment				0.0	(4.7)
Actuarial loss (gain)	[1],[2]			1.1	(37.1)
Benefits paid				(9.1)	(9.8)
Settlements	[3]			0.0	0.0
Other				2.0	2.2
Benefit obligation at end of period		100.9		102.1	102.4
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period				3.2	3.3
Actual return (loss) on plan assets				0.0	0.1
Employer contributions				6.9	7.3
Benefits paid				(7.0)	(7.5)
Settlements	[3]			0.0	0.0
Fair value of plan assets at end of period		$ 3.3		3.1	3.2
Unfunded status				$ (99.0)	$ (99.2)

[1] The actuarial loss recorded for the year ended December 31, 2023 for the pension plans was primarily driven by a decrease in the discount rate. The actuarial (gain) recorded during the year ended December 31, 2022 for the pension plans was primarily driven by an increase in the discount rate.

[2] The actuarial loss recorded for the year ended December 31, 2023 for the postretirement plans was primarily driven by a decrease in the discount rate. The actuarial (gain) recorded for the year ended December 31, 2022 for the postretirement plans was primarily driven by an increase in the discount rate, updated census data and the result of actual versus expected benefit payments.

[3] Participant elections to take lump sum payments that exceeded the settlement accounting threshold and have been categorized as settlements.