Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 33.4	$ (9.5)	$ 37.3	$ (4.1)
Net unrealized holding losses on fixed maturities:
Unrealized holding losses	(14.7)	11.0	(61.6)	65.6
Reclassification adjustments for gains realized in net income	0.0	(20.2)	(1.7)	(43.8)
Income tax benefit	3.1	3.2	13.3	(7.6)
Total net unrealized holding losses on fixed maturities	(11.6)	(6.0)	(50.0)	14.2
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.3)	(4.1)	(4.1)
Net actuarial loss	2.2	2.1	6.4	6.1
Income tax expense	(0.2)	(0.3)	(0.5)	(0.8)
Total net unrecognized benefit plan obligations	0.6	0.5	1.8	1.2
Other comprehensive loss	(11.0)	(5.5)	(48.2)	15.4
Comprehensive loss	$ 22.5	$ (15.0)	$ (10.9)	$ 11.3