Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	$ 666.6	$ 661.8
Separate account assets	136,978.9	140,072.8
Investment contracts	$ (177.4)	(176.4)
Fixed maturities valued using internal pricing models
Fixed maturities classified as Level 3 assets, percent valued using internal pricing models (as a percent)	1.00%
Fixed maturities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	$ 49,966.5	49,670.8
U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,503.5	1,121.8
Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	793.3	891.1
States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,717.1	4,204.0
Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	31,140.2	31,535.0
Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,627.5	2,822.9
Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,919.8	3,975.5
Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	667.5	504.1
Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,597.6	4,616.4
Equity securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	104.5	123.0
Recurring Fair Value Measurements
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	666.6	661.8
Other investments	517.2	478.9
Cash equivalents	1,603.2	1,041.7
Sub-total excluding separate account assets	54,747.5	53,421.0
Separate account assets	136,978.9	140,072.8
Total assets	191,726.4	193,493.8
Investment contracts	(177.4)	(176.4)
Derivative liabilities	(772.4)	(791.8)
Other liabilities	(298.4)	(310.1)
Total liabilities	(1,248.2)	(1,278.3)
Net assets (liabilities)	190,478.2	192,215.5
Recurring Fair Value Measurements | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Other investments	208.1	3.3
Cash equivalents	26.5
Sub-total excluding separate account assets	1,882.1	934.2
Separate account assets	72,303.6	73,363.4
Total assets	74,185.7	74,297.6
Net assets (liabilities)	74,185.7	74,297.6
Recurring Fair Value Measurements | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	619.4	608.1
Other investments	274.0	348.4
Cash equivalents	1,576.7	1,041.7
Sub-total excluding separate account assets	52,264.7	51,749.9
Separate account assets	57,661.4	60,818.0
Total assets	109,926.1	112,567.9
Derivative liabilities	(721.9)	(756.3)
Other liabilities	(230.3)	(243.8)
Total liabilities	(952.2)	(1,000.1)
Net assets (liabilities)	108,973.9	111,567.8
Recurring Fair Value Measurements | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	47.2	53.7
Other investments	35.1	127.2
Sub-total excluding separate account assets	600.7	736.9
Separate account assets	7,013.9	5,891.4
Total assets	7,614.6	6,628.3
Investment contracts	(177.4)	(176.4)
Derivative liabilities	(50.5)	(35.5)
Other liabilities	(68.1)	(66.3)
Total liabilities	(296.0)	(278.2)
Net assets (liabilities)	7,318.6	6,350.1
Recurring Fair Value Measurements | Fixed maturities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	49,966.5	49,670.8
Trading	686.8	604.6
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	972.2	760.8
Trading	199.2
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	48,615.5	48,497.8
Trading	352.1	464.9
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	378.8	412.2
Trading	135.5	139.7
Recurring Fair Value Measurements | U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,503.5	1,121.8
Recurring Fair Value Measurements | U.S. government and agencies | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	931.0	720.5
Recurring Fair Value Measurements | U.S. government and agencies | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	572.5	401.3
Recurring Fair Value Measurements | Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	793.3	891.1
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3.0
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	711.2	852.4
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	79.1	38.7
Recurring Fair Value Measurements | States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,717.1	4,204.0
Recurring Fair Value Measurements | States and political subdivisions | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,717.1	4,204.0
Recurring Fair Value Measurements | Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	31,140.2	31,535.0
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	38.2	40.3
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	30,878.1	31,249.1
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	223.9	245.6
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,627.5	2,822.9
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,627.5	2,822.9
Recurring Fair Value Measurements | Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,919.8	3,975.5
Recurring Fair Value Measurements | Commercial mortgage-backed securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,915.0	3,975.5
Recurring Fair Value Measurements | Commercial mortgage-backed securities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4.8
Recurring Fair Value Measurements | Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	667.5	504.1
Recurring Fair Value Measurements | Collateralized debt obligations | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	604.0	439.9
Recurring Fair Value Measurements | Collateralized debt obligations | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	63.5	64.2
Recurring Fair Value Measurements | Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,597.6	4,616.4
Recurring Fair Value Measurements | Other debt obligations | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,590.1	4,552.7
Recurring Fair Value Measurements | Other debt obligations | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	7.5	63.7
Recurring Fair Value Measurements | Equity securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	104.5	123.0
Trading	1,202.7	840.2
Recurring Fair Value Measurements | Equity securities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	62.2	64.2
Trading	413.9	105.9
Recurring Fair Value Measurements | Equity securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	38.2	54.7
Trading	788.8	734.3
Recurring Fair Value Measurements | Equity securities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	$ 4.1	$ 4.1