Income Tax Matters - Income Tax Provision (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Federal income tax (expense) benefit
Current federal tax (expense) benefit	$ 6.1	$ 5.7	$ 11.9
Deferred federal income tax (expense) benefit	(18.8)	(19.6)	(34.7)
Federal benefit (expense) applied to decrease (increase) Retained earnings/Other comprehensive income	5.1	2.7	(0.7)
Federal income tax (expense) benefit	(7.6)	(11.2)	(23.5)
Foreign income tax (expense) benefit
Current foreign tax (expense) benefit	(0.4)	(1.1)	(1.9)
Deferred foreign income tax (expense) benefit	(0.1)	(0.3)	0.1
Foreign benefit (expense) applied to decrease (increase) Retained earnings/Other comprehensive income	(0.1)	(0.1)	0.0
Foreign income tax (expense) benefit	(0.6)	(1.5)	(1.8)
State income tax (expense) benefit
Current state tax (expense) benefit	(0.6)	(1.8)	(1.5)
Deferred state income tax (expense) benefit	(2.3)	(4.5)	(1.4)
State benefit (expense) applied to decrease (increase) Retained earnings/Other comprehensive income	1.1	0.6	(0.1)
State income tax (expense) benefit	(1.8)	(5.7)	(3.0)
Total income tax (expense) benefit
Current tax (expense) benefit	5.1	2.8	8.5
Deferred income tax (expense) benefit	(21.2)	(24.4)	(36.0)
Benefit (expense) applied to decrease (increase) Retained earnings/Other comprehensive income	6.1	3.2	(0.8)
Income tax provision	$ (10.0)	$ (18.4)	$ (28.3)