Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (114.6)	$ 49.4
Net unrealized holding gain (loss) on fixed available-for-sale investments:
Unrealized holding gain	31.0	34.5
Reclassification adjustments for gains realized in net income	(2.4)	(0.1)
Income tax expense	(6.0)	(7.2)
Total net unrealized holding gain on available-for-sale investments	22.6	27.2
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.6)	(1.6)
Net actuarial loss	3.7	2.4
Income tax expense	(0.5)	(0.2)
Total net unrecognized benefit plan obligations	1.6	0.6
Other comprehensive income	24.2	27.8
Comprehensive (loss) income	$ (90.4)	$ 77.2