Accumulated Other Comprehensive Income (Loss) - Components of AOCI (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Derivatives	$ 221.0		$ 156.1
DAC/VOBA and sales inducements adjustment on available-for-sale securities	(834.6)		(562.2)
Accumulated Other Comprehensive Income (Loss), Premium Deficiency Reserve, Adjustment for Available-for-sale Securities	(117.0)		(93.8)
Unrealized capital gains (losses), before tax	1,503.0		855.5
Deferred income tax asset (liability)	(460.4)		(208.7)
Unrealized capital gains (losses), after tax	1,042.6		646.8
Pension and other post-employment benefits liability, net of tax	11.6		8.1
AOCI	1,054.2	747.5	654.9
Fixed maturities
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Fixed maturities, net of OTTI	2,221.3		1,340.5
Equity securities
Components Of Accumulated Other Comprehensive Income Loss [Line Items]
Equity securities, available-for-sale	$ 12.3		$ 14.9