Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	$ 304.0	$ 893.6
Separate account assets	159,272.7	139,832.6
Investment contracts	(160.3)	(176.5)
Unfunded commitments of investments measured using NAV	$ 46.1	57.6
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	$ 59,388.4	54,846.1
U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,351.7	1,433.0
Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	901.5	893.6
States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	6,801.6	5,569.2
Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	37,033.1	34,192.4
Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,523.3	2,834.7
Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,708.3	4,096.5
Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,359.3	758.6
Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	5,709.6	5,068.1
Equity portfolios
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	96.0	98.9
Recurring Fair Value Measurements
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	304.0	893.6
Other investments	498.0	470.0
Cash equivalents	1,793.3	1,947.1
Sub-total excluding separate account assets	64,416.3	60,067.5
Separate account assets	159,272.7	139,832.6
Total assets	223,689.0	199,900.1
Investment contracts	(160.3)	(176.5)
Derivative liabilities	(274.7)	(573.0)
Other liabilities	(253.4)	(272.2)
Total liabilities	(688.4)	(1,021.7)
Net assets (liabilities)	223,000.6	198,878.4
Recurring Fair Value Measurements | Amount measured at net asset value
Assets (liabilities) measured at fair value on a recurring basis
Other investments	85.8	92.7
Sub-total excluding separate account assets	85.8	92.7
Separate account assets	120.4
Total assets	206.2	92.7
Net assets (liabilities)	206.2	92.7
Recurring Fair Value Measurements | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Other investments	176.0	169.8
Cash equivalents	46.8	51.2
Sub-total excluding separate account assets	1,734.6	1,742.6
Separate account assets	90,090.6	79,688.1
Total assets	91,825.2	81,430.7
Net assets (liabilities)	91,825.2	81,430.7
Recurring Fair Value Measurements | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	279.8	859.7
Other investments	229.7	170.6
Cash equivalents	1,746.5	1,895.9
Sub-total excluding separate account assets	62,289.2	57,548.4
Separate account assets	61,410.3	52,789.7
Total assets	123,699.5	110,338.1
Derivative liabilities	(268.6)	(550.4)
Other liabilities	(253.4)	(212.3)
Total liabilities	(522.0)	(762.7)
Net assets (liabilities)	123,177.5	109,575.4
Recurring Fair Value Measurements | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Derivative instruments, assets	24.2	33.9
Other investments	6.5	36.9
Sub-total excluding separate account assets	306.7	683.8
Separate account assets	7,651.4	7,354.8
Total assets	7,958.1	8,038.6
Investment contracts	(160.3)	(176.5)
Derivative liabilities	(6.1)	(22.6)
Other liabilities		(59.9)
Total liabilities	(166.4)	(259.0)
Net assets (liabilities)	7,791.7	7,779.6
Recurring Fair Value Measurements | Fixed maturities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	59,388.4	54,846.1
Trading	566.0	398.4
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	920.4	1,020.7
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	58,194.7	53,308.0
Trading	566.0	305.5
Recurring Fair Value Measurements | Fixed maturities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	273.3	517.4
Trading		92.9
Recurring Fair Value Measurements | U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,351.7	1,433.0
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	896.8	996.5
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	454.9	436.5
Recurring Fair Value Measurements | Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	901.5	893.6
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	2.9	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	891.2	828.5
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	7.4	62.1
Recurring Fair Value Measurements | States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	6,801.6	5,569.2
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	6,801.6	5,569.2
Recurring Fair Value Measurements | Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	37,033.1	34,192.4
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	20.7	21.2
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	36,884.4	33,912.1
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	128.0	259.1
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,523.3	2,834.7
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	2,523.3	2,834.7
Recurring Fair Value Measurements | Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,708.3	4,096.5
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	3,697.7	4,025.4
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	10.6	71.1
Recurring Fair Value Measurements | Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	1,359.3	758.6
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	1,234.3	725.0
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	125.0	33.6
Recurring Fair Value Measurements | Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	5,709.6	5,068.1
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	5,707.3	4,976.6
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	2.3	91.5
Recurring Fair Value Measurements | Equity portfolios
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	96.0	98.9
Trading	1,770.6	1,413.4
Recurring Fair Value Measurements | Equity portfolios | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	47.4	55.2
Trading	544.0	445.7
Recurring Fair Value Measurements | Equity portfolios | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	45.9	41.0
Trading	1,226.6	967.7
Recurring Fair Value Measurements | Equity portfolios | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	$ 2.7	$ 2.7