Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 51.2	$ 107.4	$ 60.8
Net unrealized holding (losses) gains on investments:
Unrealized holding (losses) gains arising during year	(39.2)	59.8	(27.1)
Reclassification adjustments for gains realized in net income	(24.7)	(20.7)	(23.2)
Income tax benefit (expense)	22.4	(13.7)	10.9
Total net unrealized holding (losses) gains on investments	(41.5)	25.4	(39.4)
Amortization of gain on derivative used in cash flow hedge	0.0	0.0	(0.1)
Net unrecognized benefit plan obligations:
Net actuarial gain (loss) arising during period	5.3	(54.4)	32.5
Reclassification adjustments for amortization to statements of income:
Negative prior service cost	(5.4)	(5.5)	(5.5)
Net actuarial loss	11.5	6.9	9.1
Income tax (expense) benefit	(4.0)	18.5	0.0
Total net unrecognized benefit plan obligations	7.4	(34.5)	36.1
Other comprehensive loss	(34.1)	(9.1)	(3.4)
Comprehensive income	$ 17.1	$ 98.3	$ 57.4