Fair Value Measurements (Details 7) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets (liabilities)
Mortgage loans	$ 11,577.9	$ 11,519.7
Policy loans	866.5	864.9
Short-term debt	(46.5)	(40.8)
Long-term debt	(2,790.5)	(2,671.3)
Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	903.8	2,364.6
Bank deposits	(1,433.4)	(1,404.4)
Cash collateral payable	(75.0)	(205.6)
Fair value hierarchy Level 2
Assets (liabilities)
Other investments	146.0	195.3
Cash and cash equivalents not required to be reported at fair value		40.0
Investment-type insurance contracts	(6,822.7)	(7,367.4)
Short-term debt	(46.5)	(40.8)
Long-term debt	(3,057.3)	(2,921.7)
Bank deposits	(792.1)	(773.3)
Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	12,350.1	12,163.7
Policy loans	1,051.9	1,056.8
Other investments	87.2	85.2
Investment-type insurance contracts	(24,583.2)	(25,164.2)
Long-term debt	(33.1)	(29.7)
Separate account liabilities	(110,268.8)	(72,173.8)
Carrying amount
Assets (liabilities)
Mortgage loans	11,577.9	11,519.7
Policy loans	866.5	864.9
Other investments	231.7	280.1
Cash and cash equivalents not required to be reported at fair value	903.8	2,404.6
Investment-type insurance contracts	(30,791.6)	(31,953.1)
Short-term debt	(46.5)	(40.8)
Long-term debt	(2,790.5)	(2,671.3)
Separate account liabilities	(111,716.3)	(73,096.0)
Bank deposits	(2,181.6)	(2,174.7)
Cash collateral payable	(75.0)	(205.6)
Fair Value
Assets (liabilities)
Mortgage loans	12,350.1	12,163.7
Policy loans	1,051.9	1,056.8
Other investments	233.2	280.5
Cash and cash equivalents not required to be reported at fair value	903.8	2,404.6
Investment-type insurance contracts	(31,405.9)	(32,531.6)
Short-term debt	(46.5)	(40.8)
Long-term debt	(3,090.4)	(2,951.4)
Separate account liabilities	(110,268.8)	(72,173.8)
Bank deposits	(2,225.5)	(2,177.7)
Cash collateral payable	$ (75.0)	$ (205.6)