Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	$ 1,063.8	$ 666.6
Separate account assets	132,325.9	136,978.9
Investment contracts	$ (213.0)	(177.4)
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	$ 53,045.4	49,966.5
U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,621.8	1,503.5
Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	886.5	793.3
States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	5,149.3	4,717.1
Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	33,132.6	31,140.2
Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,627.6	2,627.5
Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,179.6	3,919.8
Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	752.0	667.5
Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,696.0	4,597.6
Equity securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	103.7	104.5
Net Asset Value Measurements
Assets (liabilities) measured at fair value on a recurring basis
Unfunded commitments	37.5	7.3
Recurring Fair Value Measurements
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	1,063.8	666.6
Other investments	741.9	517.2
Cash equivalents	1,477.9	1,603.2
Sub-total excluding separate account assets	58,382.2	54,747.5
Separate account assets	132,325.9	136,978.9
Total assets	190,708.1	191,726.4
Investment contracts	(213.0)	(177.4)
Derivative liabilities	(890.8)	(772.4)
Other liabilities	(286.3)	(298.4)
Total liabilities	(1,390.1)	(1,248.2)
Net assets (liabilities)	189,318.0	190,478.2
Recurring Fair Value Measurements | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Other investments	393.1	208.1
Cash equivalents	52.3	26.5
Sub-total excluding separate account assets	2,199.0	1,882.1
Separate account assets	73,642.3	72,303.6
Total assets	75,841.3	74,185.7
Net assets (liabilities)	75,841.3	74,185.7
Recurring Fair Value Measurements | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	1,004.7	619.4
Other investments	222.8	204.4
Cash equivalents	1,425.6	1,576.7
Sub-total excluding separate account assets	55,465.5	52,195.1
Separate account assets	51,589.1	57,661.4
Total assets	107,054.6	109,856.5
Derivative liabilities	(854.7)	(721.9)
Other liabilities	(213.2)	(230.3)
Total liabilities	(1,067.9)	(952.2)
Net assets (liabilities)	105,986.7	108,904.3
Recurring Fair Value Measurements | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	59.1	47.2
Other investments	35.9	35.1
Sub-total excluding separate account assets	627.6	600.7
Separate account assets	7,094.5	7,013.9
Total assets	7,722.1	7,614.6
Investment contracts	(213.0)	(177.4)
Derivative liabilities	(36.1)	(50.5)
Other liabilities	(73.1)	(68.1)
Total liabilities	(322.2)	(296.0)
Net assets (liabilities)	7,399.9	7,318.6
Recurring Fair Value Measurements | Fixed maturities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	53,045.4	49,966.5
Trading	687.5	686.8
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,046.8	972.2
Trading	203.4	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	51,605.9	48,615.5
Trading	348.3	352.1
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	392.7	378.8
Trading	135.8	135.5
Recurring Fair Value Measurements | U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,621.8	1,503.5
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	1,005.6	931.0
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	616.2	572.5
Recurring Fair Value Measurements | Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	886.5	793.3
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	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	785.7	711.2
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	97.8	79.1
Recurring Fair Value Measurements | States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	5,149.3	4,717.1
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	5,149.3	4,717.1
Recurring Fair Value Measurements | Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	33,132.6	31,140.2
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	38.2
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	32,871.7	30,878.1
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	222.7	223.9
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,627.6	2,627.5
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	2,627.6	2,627.5
Recurring Fair Value Measurements | Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,179.6	3,919.8
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	4,177.2	3,915.0
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	2.4	4.8
Recurring Fair Value Measurements | Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	752.0	667.5
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	689.2	604.0
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	62.8	63.5
Recurring Fair Value Measurements | Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	4,696.0	4,597.6
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	4,689.0	4,590.1
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	7.0	7.5
Recurring Fair Value Measurements | Equity securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	103.7	104.5
Trading	1,262.0	1,202.7
Recurring Fair Value Measurements | Equity securities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	62.8	62.2
Trading	440.6	413.9
Recurring Fair Value Measurements | Equity securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	36.8	38.2
Trading	821.4	788.8
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
Recurring Fair Value Measurements | Net Asset Value Measurements
Assets (liabilities) measured at fair value on a recurring basis
Other investments	90.1	69.6
Sub-total excluding separate account assets	90.1	69.6
Total assets	90.1	69.6
Net assets (liabilities)	$ 90.1	$ 69.6