Accumulated Other Comprehensive Income (Loss) - Components of AOCI (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Derivatives	$ 215.3		$ 201.0
DAC/VOBA and sales inducements adjustment on available-for-sale securities	(608.7)		(372.0)
Premium deficiency reserve adjustment	(112.0)		(94.8)
Unrealized capital gains (losses), before tax	1,363.7		715.9
Deferred income tax asset (liability)	(350.9)		(126.9)
Unrealized capital gains (losses), after tax	1,012.8		589.0
Pension and other post-employment benefits liability, net of tax	6.2		7.7
AOCI	1,019.0	$ 386.8	596.7
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, net of OTTI	1,853.2		967.0
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Equity securities, available-for-sale	$ 15.9		$ 14.7