Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Amounts Reclassified Out Of Accumulated Other Comprehensive Income Loss [Abstract]
Net income	$ 3.0	$ 6.2	$ 30.1	$ 25.9
Net unrealized holding gains (losses) on investments:
Unrealized holding gains (losses) arising during the period ended	34.9	(67.1)	57.9	(47.7)
Reclassification adjustments for gains realized in net income	(5.3)	(1.5)	(16.0)	(8.2)
Income tax (expense) benefit	(3.5)	7.3	(7.7)	7.6
Total net unrealized holding gains (losses) on investments	26.1	(61.3)	34.2	(48.3)
Amortization of gain on derivative used in cash flow hedge	0	(0.1)	0	(0.1)
Net actuarial loss arising during period	(1.6)	0	(1.6)	0
Reclassification adjustments for amortization to statements of income:
Prior service benefits	(1.4)	(1.4)	(2.8)	(2.8)
Net actuarial loss	2.2	2.3	3.8	4.6
Total net unrecognized benefit plan obligations	(0.8)	0.9	(0.6)	1.8
Other comprehensive income (loss), net of tax	25.3	(60.5)	33.6	(46.6)
Comprehensive income (loss), net of tax	$ 28.3	$ (54.3)	$ 63.7	$ (20.7)