Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 8.7	$ (24.6)	$ 4.6	$ (21.6)
Net unrealized holding gains on investments:
Unrealized holding gains	23.8	38.2	54.6	75.2
Reclassification adjustments for gains realized in net income	(15.8)	(6.5)	(23.6)	(7.8)
Income tax expense	(2.8)	(11.1)	(10.8)	(23.6)
Total net unrealized holding gains on investments	5.2	20.6	20.2	43.8
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(1.4)	(1.4)	(2.8)	(2.8)
Net actuarial loss	1.9	2.3	4.0	4.7
Income tax expense	(0.2)	(0.2)	(0.5)	(0.6)
Total net unrecognized benefit plan obligations	0.3	0.7	0.7	1.3
Other comprehensive income	5.5	21.3	20.9	45.1
Comprehensive income	$ 14.2	$ (3.3)	$ 25.5	$ 23.5