Fair Value Measurements (Details 7) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets (liabilities)
Mortgage loans	$ 11,478.4	$ 11,533.6
Policy loans	843.8	859.7
Short-term debt	(148.8)	(150.6)
Long-term debt	(2,516.0)	(2,601.4)
Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	811.8	912.8
Bank deposits	(1,241.6)	(1,252.2)
Cash collateral payable	(37.6)	(32.5)
Fair value hierarchy Level 2
Assets (liabilities)
Other investments	156.5	140.9
Investment-type insurance contracts	(5,708.5)	(5,902.2)
Short-term debt	(148.8)	(150.6)
Long-term debt	(2,610.8)	(2,639.0)
Bank deposits	(676.1)	(698.9)
Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	11,795.6	11,773.5
Policy loans	995.1	963.3
Other investments	28.1	33.8
Investment-type insurance contracts	(24,055.9)	(24,190.9)
Long-term debt	(67.0)	(53.1)
Separate account liabilities	(120,613.8)	(118,059.7)
Carrying amount
Assets (liabilities)
Mortgage loans	11,478.4	11,533.6
Policy loans	843.8	859.7
Other investments	183.9	174.0
Cash and cash equivalents not required to be reported at fair value	811.8	912.8
Investment-type insurance contracts	(29,602.0)	(29,909.6)
Short-term debt	(148.8)	(150.6)
Long-term debt	(2,516.0)	(2,601.4)
Separate account liabilities	(122,037.3)	(119,500.7)
Bank deposits	(1,909.9)	(1,949.0)
Cash collateral payable	(37.6)	(32.5)
Fair Value
Assets (liabilities)
Mortgage loans	11,795.6	11,773.5
Policy loans	995.1	963.3
Other investments	184.6	174.7
Cash and cash equivalents not required to be reported at fair value	811.8	912.8
Investment-type insurance contracts	(29,764.4)	(30,093.1)
Short-term debt	(148.8)	(150.6)
Long-term debt	(2,677.9)	(2,692.1)
Separate account liabilities	(120,613.8)	(118,059.7)
Bank deposits	(1,917.7)	(1,951.1)
Cash collateral payable	$ (37.6)	$ (32.5)