Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (9.5)	$ 10.1	$ (4.9)	$ (11.5)
Net unrealized holding gains on investments:
Unrealized holding gains	11.0	6.3	65.6	81.5
Reclassification adjustments for gains realized in net income	(20.2)	(8.8)	(43.8)	(16.6)
Income tax expense	3.2	0.8	(7.6)	(22.8)
Total net unrealized holding gains on investments	(6.0)	(1.7)	14.2	42.1
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.3)	(1.3)	(4.1)	(4.1)
Net actuarial loss	2.1	2.4	6.1	7.1
Income tax expense	(0.3)	(0.4)	(0.8)	(1.0)
Total net unrecognized benefit plan obligations	0.5	0.7	1.2	2.0
Other comprehensive income	(5.5)	(1.0)	15.4	44.1
Comprehensive income	$ (15.0)	$ 9.1	$ 10.5	$ 32.6