Fair Value Measurements (Details 7) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets (liabilities)
Mortgage loans	$ 12,121.7	$ 11,519.7
Policy loans	865.3	864.9
Short-term debt	(175.3)	(40.8)
Long-term debt	(2,578.6)	(2,671.3)
Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	687.8	2,364.6
Bank deposits	(1,426.8)	(1,404.4)
Cash collateral payable	(55.8)	(205.6)
Fair value hierarchy Level 2
Assets (liabilities)
Other investments	132.4	195.3
Cash and cash equivalents not required to be reported at fair value		40.0
Investment-type insurance contracts	(6,447.9)	(7,367.4)
Short-term debt	(175.3)	(40.8)
Long-term debt	(2,688.3)	(2,921.7)
Bank deposits	(727.6)	(773.3)
Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	12,492.9	12,163.7
Policy loans	1,016.7	1,056.8
Other investments	93.2	85.2
Investment-type insurance contracts	(24,113.2)	(25,164.2)
Long-term debt	(30.1)	(29.7)
Separate account liabilities	(108,022.1)	(72,173.8)
Carrying amount
Assets (liabilities)
Mortgage loans	12,121.7	11,519.7
Policy loans	865.3	864.9
Other investments	223.7	280.1
Cash and cash equivalents not required to be reported at fair value	687.8	2,404.6
Investment-type insurance contracts	(30,303.8)	(31,953.1)
Short-term debt	(175.3)	(40.8)
Long-term debt	(2,578.6)	(2,671.3)
Separate account liabilities	(109,414.6)	(73,096.0)
Bank deposits	(2,150.9)	(2,174.7)
Cash collateral payable	(55.8)	(205.6)
Fair Value
Assets (liabilities)
Mortgage loans	12,492.9	12,163.7
Policy loans	1,016.7	1,056.8
Other investments	225.6	280.5
Cash and cash equivalents not required to be reported at fair value	687.8	2,404.6
Investment-type insurance contracts	(30,561.1)	(32,531.6)
Short-term debt	(175.3)	(40.8)
Long-term debt	(2,718.4)	(2,951.4)
Separate account liabilities	(108,022.1)	(72,173.8)
Bank deposits	(2,154.4)	(2,177.7)
Cash collateral payable	$ (55.8)	$ (205.6)