Accumulated Other Comprehensive Income (Loss) - Components of AOCI (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Derivatives	$ 215.2	$ 173.7	$ 0.5
DAC/VOBA and sales inducements adjustment on available-for-sale securities	(810.6)	(603.6)	(362.4)
Premium deficiency reserve adjustment	(152.6)	(64.8)	(61.0)
Other investments	0	0	0.1
Unrealized capital gains (losses), before tax	1,456.4	1,037.1	532.0
Deferred income tax asset (liability)	(444.6)	(302.3)	(149.3)
Unrealized capital gains (losses), after tax	1,011.8	734.8	382.7
Pension and other post-employment benefits liability, net of tax	11.2	12.7	7.8
AOCI	1,023.0	747.5	390.5
Fixed maturities
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Fixed maturities, net of OTTI	2,190.9	1,518.7	933.8
Equity securities
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Equity securities, available-for-sale	$ 13.5	$ 13.1	$ 21.0