Fair Value Measurements (Details 7) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Assets (liabilities)
Mortgage loans	$ 11,158.6	$ 10,727.2
Policy loans	867.8	885.1
Short-term debt	(262.9)	(105.2)
Long-term debt	(1,576.9)	(1,564.8)
Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	791.9
Bank deposits	(1,324.1)
Cash collateral payable	(279.1)
Fair value hierarchy Level 2
Assets (liabilities)
Other investments	158.8
Investment-type insurance contracts	(6,727.7)
Short-term debt	(262.9)
Long-term debt	(1,789.7)
Bank deposits	(813.8)
Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	11,706.9
Policy loans	1,079.0
Other investments	86.5
Investment-type insurance contracts	(24,249.5)
Long-term debt	(29.6)
Separate account liabilities	(67,070.3)
Carrying amount
Assets (liabilities)
Mortgage loans	11,158.6	10,727.2
Policy loans	867.8	885.1
Other investments	244.5	165.6
Cash and cash equivalents not required to be reported at fair value	791.9	1,174.1
Investment-type insurance contracts	(31,215.0)	(32,408.5)
Short-term debt	(262.9)	(105.2)
Long-term debt	(1,576.9)	(1,564.8)
Separate account liabilities	(68,014.8)	(64,016.2)
Bank deposits	(2,132.1)	(2,142.8)
Cash collateral payable	(279.1)	(234.0)
Fair Value
Assets (liabilities)
Mortgage loans	11,706.9	11,223.4
Policy loans	1,079.0	1,114.2
Other investments	245.3	165.6
Cash and cash equivalents not required to be reported at fair value	791.9	1,174.1
Investment-type insurance contracts	(30,977.2)	(32,234.0)
Short-term debt	(262.9)	(105.2)
Long-term debt	(1,819.3)	(1,750.7)
Separate account liabilities	(67,070.3)	(62,906.9)
Bank deposits	(2,137.9)	(2,150.2)
Cash collateral payable	$ (279.1)	$ (234.0)