Income Tax Matters - Income Tax (Provision) Benefit (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Federal income tax (expense) benefit
Deferred federal income tax (expense) benefit	$ (11.6)	$ 8.9	$ 2.1
Federal benefit (expense) applied to decrease (increase) AOCI	2.5	1.7	3.7
Federal income tax (provision) benefit	(9.1)	10.6	5.8
Foreign income tax (expense) benefit
Current foreign tax (expense) benefit	(2.2)	(1.1)	(2.4)
Deferred foreign income tax (expense) benefit	0.1	(0.8)	0.5
Foreign benefit (expense) applied to decrease (increase) AOCI	0.1	0.2	0.3
Foreign income tax (provision) benefit	(2.0)	(1.7)	(1.6)
State income tax (expense) benefit
Current state tax (expense) benefit	0.6	(2.5)	(3.5)
Deferred state income tax (expense) benefit	1.0	1.6	4.2
State benefit (expense) applied to decrease (increase) AOCI	0.4	0.3	0.6
State income tax (provision) benefit	2.0	(0.6)	1.3
Total income tax (expense) benefit
Current tax (expense) benefit	(1.6)	(3.6)	(5.9)
Deferred income tax (expense) benefit	(10.5)	9.7	6.8
Benefit (expense) applied to decrease (increase) AOCI	3.0	2.2	4.6
Income tax (provision) benefit	$ (9.1)	$ 8.3	$ 5.5