Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]
Net income	$ 11.9	$ 18.5	$ 42.0	$ 44.4
Net unrealized holding (losses) gains on investments:
Unrealized holding (losses) gains	(16.3)	12.1	41.6	(35.6)
Reclassification adjustments for gains realized in net income	(1.6)	(7.5)	(17.6)	(15.7)
Income tax (expense) benefit	(0.6)	1.4	(8.3)	9.0
Total net unrealized holding (losses) gains on investments	(18.5)	6.0	15.7	(42.3)
Amortization of gain on derivative used in cash flow hedge			0	(0.1)
Net actuarial loss arising during period			(1.6)	0
Reclassification adjustments for amortization to statements of income:
Prior service benefits	(1.3)	(1.3)	(4.1)	(4.1)
Net actuarial loss	1.6	2.2	5.4	6.8
Total net unrecognized benefit plan obligations	0.3	0.9	(0.3)	2.7
Other comprehensive (loss) income, net of tax	(18.2)	6.9	15.4	(39.7)
Comprehensive (loss) income, net of tax	$ (6.3)	$ 25.4	$ 57.4	$ 4.7