Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 11.5	$ 33.4	$ 54.7	$ 37.3
Net unrealized holding gain (loss) on fixed available-for-sale investments:
Unrealized holding gain (loss)	22.6	(14.7)	91.8	(61.6)
Reclassification adjustments for gains realized in net income	(1.2)	0.0	(2.8)	(1.7)
Income tax (expense) benefit	(4.5)	3.1	(18.7)	13.3
Total net unrealized holding gain (loss) on available-for-sale investments	16.9	(11.6)	70.3	(50.0)
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.6)	(1.6)	(4.8)	(4.8)
Net actuarial loss	2.4	3.3	7.2	9.9
Income tax expense	(0.2)	(0.4)	(0.5)	(1.1)
Total net unrecognized benefit plan obligations	0.6	1.3	1.9	4.0
Other comprehensive income (loss)	17.5	(10.3)	72.2	(46.0)
Comprehensive income (loss)	$ 29.0	$ 23.1	$ 126.9	$ (8.7)