Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 21.0	$ 51.2	$ 107.4
Net change in unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during year	27.8	(39.2)	59.8
Reclassification adjustments for gains realized in net income	(36.5)	(24.7)	(20.7)
Income tax benefit (expense)	3.0	22.4	(13.7)
Total change in net unrealized holding gains (losses) on investments	(5.7)	(41.5)	25.4
Net unrecognized benefit plan obligations:
Net actuarial (loss) gain arising during period	(3.0)	5.3	(54.4)
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(5.5)	(5.4)	(5.5)
Net actuarial loss	9.4	11.5	6.9
Income tax (expense) benefit	(0.3)	(4.0)	18.5
Total net unrecognized benefit plan obligations	0.6	7.4	(34.5)
Other comprehensive loss	(5.1)	(34.1)	(9.1)
Comprehensive income	$ 15.9	$ 17.1	$ 98.3