Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (4.1)	$ 3.0
Net unrealized holding gains on investments:
Unrealized holding gains	30.8	37.0
Reclassification adjustments for gains realized in net income	(7.8)	(1.3)
Income tax expense	(8.0)	(12.5)
Total net unrealized holding gains on investments	15.0	23.2
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.4)
Net actuarial loss	2.1	2.4
Income tax expense	(0.3)	(0.4)
Total net unrecognized benefit plan obligations	0.4	0.6
Other comprehensive income	15.4	23.8
Comprehensive income	$ 11.3	$ 26.8