Accumulated Other Comprehensive Income (Loss) - Changes in AOCI (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Derivatives	$ 41.5	$ 173.2	$ 0.5
DAC/VOBA and sales inducements adjustment on available-for-sale securities	(207.0)	(241.2)	(295.3)
Premium deficiency reserve adjustment	(87.8)	(3.8)	(61.0)
Other investments	0	(0.1)	0.1
Change in unrealized gains/losses on securities, before tax	408.7	483.8	465.6
Deferred income tax asset/liability	(138.6)	(145.5)	(82.2)
Change in unrealized gains/losses on securities, after tax	270.1	338.3	383.4
Change in OTTI, before tax	10.6	21.3	(12.7)
Deferred income tax asset/liability	(3.7)	(7.5)	4.4
Change in OTTI, after tax	6.9	13.8	(8.3)
Pension and other post-employment benefit liability, before tax	(2.2)	7.6	(1.4)
Deferred income tax asset/liability	0.7	(2.7)	0.5
Pension and other post-employment benefit liability, after tax	(1.5)	4.9	(0.9)
Other comprehensive income (loss), after tax	275.5	357.0	374.2
Fixed maturities
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized capital gains (losses)	661.6	563.6	813.1
Equity securities
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Unrealized capital gains (losses)	$ 0.4	$ (7.9)	$ 8.2