Accumulated Other Comprehensive Income (Loss) - Components of AOCI (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Equity [Abstract]
Fixed maturities, net of OTTI	$ 3,680.4		$ 5,977.5
Equity securities, available-for-sale	30.6		31.5
Derivatives	236.4		178.1
DAC/VOBA adjustment on available-for-sale securities	(1,227.6)		(1,848.2)
Sales inducements adjustment on available-for-sale securities	(51.8)		(88.7)
Other	(31.5)		(30.4)
Unrealized capital gains (losses), before tax	2,636.5		4,219.8
Deferred income tax asset (liability)	(564.2)		(1,113.0)
Unrealized capital gains (losses), after tax	2,072.3		3,106.8
Pension and other post-employment benefits liability, net of tax	36.9		45.9
AOCI	$ 2,109.2	$ 3,103.7	$ 3,152.7