Fair Value Measurements (Details 7) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Assets (liabilities)
Mortgage loans	$ 11,297.9	$ 10,727.2
Policy loans	866.6	885.1
Short-term debt	(28.5)	(105.2)
Long-term debt	(2,180.0)	(1,564.8)
Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	878.3
Bank deposits	(1,364.8)
Cash collateral payable	(226.8)
Fair value hierarchy Level 2
Assets (liabilities)
Other investments	163.7
Investment-type insurance contracts	(7,421.2)
Short-term debt	(28.5)
Long-term debt	(2,470.3)
Bank deposits	(797.9)
Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	11,880.4
Policy loans	1,080.4
Other investments	87.4
Investment-type insurance contracts	(24,718.4)
Long-term debt	(29.6)
Separate account liabilities	(70,867.3)
Carrying amount
Assets (liabilities)
Mortgage loans	11,297.9	10,727.2
Policy loans	866.6	885.1
Other investments	250.1	165.6
Cash and cash equivalents not required to be reported at fair value	878.3	1,174.1
Investment-type insurance contracts	(31,633.5)	(32,408.5)
Short-term debt	(28.5)	(105.2)
Long-term debt	(2,180.0)	(1,564.8)
Separate account liabilities	(71,784.0)	(64,016.2)
Bank deposits	(2,157.7)	(2,142.8)
Cash collateral payable	(226.8)	(234.0)
Fair Value
Assets (liabilities)
Mortgage loans	11,880.4	11,223.4
Policy loans	1,080.4	1,343.4
Other investments	251.1	165.6
Cash and cash equivalents not required to be reported at fair value	878.3	1,174.1
Investment-type insurance contracts	(32,139.6)	(32,234.0)
Short-term debt	(28.5)	(105.2)
Long-term debt	(2,499.9)	(1,750.7)
Separate account liabilities	(70,867.3)	(62,906.9)
Bank deposits	(2,162.7)	(2,150.2)
Cash collateral payable	$ (226.8)	$ (234.0)