Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (10.7)	$ 21.0	$ 51.2
Net change in unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during year	69.0	27.8	(39.2)
Reclassification adjustments for gains realized in net income	(65.1)	(36.5)	(24.7)
Income tax (expense) benefit	(0.7)	3.0	22.4
Total change in net unrealized holding gains (losses) on investments	3.2	(5.7)	(41.5)
Net unrecognized benefit plan obligations:
Net actuarial (loss) gain arising during period	(1.0)	(3.0)	5.3
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(5.5)	(5.5)	(5.4)
Net actuarial gain	8.0	9.4	11.5
Income tax expense	(0.5)	(0.3)	(4.0)
Total net unrecognized benefit plan obligations	1.0	0.6	7.4
Other comprehensive income (loss)	4.2	(5.1)	(34.1)
Comprehensive (loss) income	$ (6.5)	$ 15.9	$ 17.1