Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	$ 65,268.1	$ 60,108.5
Fixed maturities, trading	686.8	636.1
Equity securities	1,895.2	1,843.7
Derivative instruments, assets	291.5	194.0
Separate account assets	162,848.6	144,987.9
Investment contracts	$ (118.7)	(45.2)
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%
Amount measured at net asset value
Assets (liabilities) measured at fair value on a recurring basis
Unfunded commitments of investments measured using NAV	$ 24.7	32.0
U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	1,536.4	1,441.0
Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	1,000.7	890.5
States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	6,968.6	6,225.7
Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	38,171.3	35,539.3
Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	2,437.0	2,460.6
Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	4,569.9	3,945.6
Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	2,759.4	2,420.8
Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	7,824.8	7,185.0
Recurring Fair Value Measurements
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	65,268.1	60,108.5
Fixed maturities, trading	686.8	636.1
Equity securities	1,895.2	1,843.7
Derivative instruments, assets	291.5	194.0
Other investments	812.5	828.6
Cash equivalents	2,123.6	1,775.9
Sub-total excluding separate account assets	71,077.7	65,386.8
Separate account assets	162,848.6	144,987.9
Total assets	233,926.3	210,374.7
Investment contracts	(118.7)	(45.2)
Derivative liabilities	(168.0)	(157.1)
Other liabilities	(96.0)	(91.5)
Total liabilities	(382.7)	(293.8)
Net assets (liabilities)	233,543.6	210,080.9
Recurring Fair Value Measurements | Amount measured at net asset value
Assets (liabilities) measured at fair value on a recurring basis
Other investments	84.2	75.4
Sub-total excluding separate account assets	84.2	75.4
Separate account assets	129.7	124.6
Total assets	213.9	200.0
Net assets (liabilities)	213.9	200.0
Recurring Fair Value Measurements | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	1,146.7	1,049.7
Equity securities	633.5	661.2
Other investments	201.7	125.9
Cash equivalents	36.6	46.3
Sub-total excluding separate account assets	2,018.5	1,883.1
Separate account assets	91,789.6	79,572.0
Total assets	93,808.1	81,455.1
Net assets (liabilities)	93,808.1	81,455.1
Recurring Fair Value Measurements | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	63,965.0	58,920.0
Fixed maturities, trading	686.3	636.1
Equity securities	1,261.7	1,182.5
Derivative instruments, assets	261.3	175.4
Other investments	497.5	610.1
Cash equivalents	2,087.0	1,729.6
Sub-total excluding separate account assets	68,758.8	63,253.7
Separate account assets	62,239.5	56,675.8
Total assets	130,998.3	119,929.5
Derivative liabilities	(153.6)	(141.6)
Other liabilities	(96.0)	(91.5)
Total liabilities	(249.6)	(233.1)
Net assets (liabilities)	130,748.7	119,696.4
Recurring Fair Value Measurements | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	156.4	138.8
Fixed maturities, trading	0.5
Derivative instruments, assets	30.2	18.6
Other investments	29.1	17.2
Sub-total excluding separate account assets	216.2	174.6
Separate account assets	8,689.8	8,615.5
Total assets	8,906.0	8,790.1
Investment contracts	(118.7)	(45.2)
Derivative liabilities	(14.4)	(15.5)
Total liabilities	(133.1)	(60.7)
Net assets (liabilities)	8,772.9	8,729.4
Recurring Fair Value Measurements | U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	1,536.4	1,441.0
Recurring Fair Value Measurements | U.S. government and agencies | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	1,125.6	1,027.8
Recurring Fair Value Measurements | U.S. government and agencies | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	410.8	413.2
Recurring Fair Value Measurements | Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	1,000.7	890.5
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	2.0	2.7
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	994.8	883.2
Recurring Fair Value Measurements | Non-U.S. governments | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	3.9	4.6
Recurring Fair Value Measurements | States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	6,968.6	6,225.7
Recurring Fair Value Measurements | States and political subdivisions | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	6,968.6	6,225.7
Recurring Fair Value Measurements | Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	38,171.3	35,539.3
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	19.1	19.2
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	38,107.5	35,462.2
Recurring Fair Value Measurements | Corporate debt securities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	44.7	57.9
Recurring Fair Value Measurements | Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	2,437.0	2,460.6
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
Fixed maturities, available-for-sale	2,437.0	2,460.6
Recurring Fair Value Measurements | Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	4,569.9	3,945.6
Recurring Fair Value Measurements | Commercial mortgage-backed securities | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	4,556.8	3,936.1
Recurring Fair Value Measurements | Commercial mortgage-backed securities | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	13.1	9.5
Recurring Fair Value Measurements | Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	2,759.4	2,420.8
Recurring Fair Value Measurements | Collateralized debt obligations | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	2,724.6	2,412.5
Recurring Fair Value Measurements | Collateralized debt obligations | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	34.8	8.3
Recurring Fair Value Measurements | Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	7,824.8	7,185.0
Recurring Fair Value Measurements | Other debt obligations | Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	7,764.9	7,126.5
Recurring Fair Value Measurements | Other debt obligations | Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Fixed maturities, available-for-sale	$ 59.9	$ 58.5