Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ 11.6	$ 11.5	$ (68.7)	$ 54.7
Net unrealized holding gains on fixed available-for-sale investments:
Unrealized holding gains	4.6	22.6	74.5	91.8
Reclassification adjustments for gains realized in net income	0.2	(1.2)	(3.2)	(2.8)
Income tax expense	(1.1)	(4.5)	(15.0)	(18.7)
Total net unrealized holding gains on available-for-sale investments	3.7	16.9	56.3	70.3
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.6)	(1.6)	(4.8)	(4.8)
Net actuarial loss	3.7	2.4	11.0	7.2
Income tax expense	(0.4)	(0.2)	(1.3)	(0.5)
Total net unrecognized benefit plan obligations	1.7	0.6	4.9	1.9
Other comprehensive income	5.4	17.5	61.2	72.2
Comprehensive (loss) income	$ 17.0	$ 29.0	$ (7.5)	$ 126.9