Fair Value of Financial Instruments - Schedule of Changes in Balances of Level 3 Financial Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Changes in balances of Level 3 financial assets
Balance at beginning of period	$ 763.0	$ 701.6
Net earnings (loss)	0.0	0.0
Other comp. income (loss)	(130.5)	23.5
Purchases and issuances	92.7	55.6
Sales and settlements	(53.7)	(78.9)
Transfer to Level 3	62.5	110.6
Transfer out of Level 3	(182.0)	(5.7)
Balance at end of period	552.0	806.7
Changes in balances of Level 3 financial liabilities
Balance at beginning of period	7.8	11.9
Net earnings (loss)	(3.4)	(3.1)
Other comp. income (loss)	0.0	0.0
Purchases and issuances	0.0	0.0
Sales and settlements	0.0	0.0
Transfer to Level 3	0.0	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	4.4	8.8
Structured securities
Changes in balances of Level 3 financial assets
Transfer to Level 3	119.5
Transfer out of Level 3	(104.9)
Total fixed maturity securities
Changes in balances of Level 3 financial assets
Balance at beginning of period	705.4	640.4
Net earnings (loss)	0.0	0.0
Other comp. income (loss)	(114.5)	23.5
Purchases and issuances	92.7	55.6
Sales and settlements	(53.7)	(78.9)
Transfer to Level 3	62.5	110.6
Transfer out of Level 3	(182.0)	(5.7)
Balance at end of period	510.4	745.5
States, municipalities and political subdivisions
Changes in balances of Level 3 financial assets
Balance at beginning of period	0.0
Net earnings (loss)	0.1
Other comp. income (loss)	(1.0)
Purchases and issuances	0.0
Sales and settlements	0.0
Transfer to Level 3	9.9
Transfer out of Level 3	0.0
Balance at end of period	9.0
Residential mortgage-backed securities
Changes in balances of Level 3 financial assets
Balance at beginning of period	9.2	19.0
Net earnings (loss)	0.0	0.0
Other comp. income (loss)	(1.9)	0.1
Purchases and issuances	0.0	0.0
Sales and settlements	(0.7)	(0.3)
Transfer to Level 3	6.8	0.0
Transfer out of Level 3	(0.4)	(5.7)
Balance at end of period	13.0	13.1
Commercial mortgage-backed securities
Changes in balances of Level 3 financial assets
Balance at beginning of period	34.6	58.2
Net earnings (loss)	0.0	0.0
Other comp. income (loss)	(5.3)	1.6
Purchases and issuances	0.0	2.4
Sales and settlements	(0.2)	(0.4)
Transfer to Level 3	5.1	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	34.2	61.8
Asset-backed securities
Changes in balances of Level 3 financial assets
Balance at beginning of period	550.6	478.2
Net earnings (loss)	0.0	0.0
Other comp. income (loss)	(93.3)	13.3
Purchases and issuances	60.0	48.6
Sales and settlements	(51.5)	(73.6)
Transfer to Level 3	0.0	5.6
Transfer out of Level 3	(180.3)	0.0
Balance at end of period	285.5	472.1
Corporate and other
Changes in balances of Level 3 financial assets
Balance at beginning of period	111.0	85.0
Net earnings (loss)	(0.1)	0.0
Other comp. income (loss)	(13.0)	8.5
Purchases and issuances	32.7	4.6
Sales and settlements	(1.3)	(4.6)
Transfer to Level 3	40.7	105.0
Transfer out of Level 3	(1.3)	0.0
Balance at end of period	168.7	198.5
Equity securities
Changes in balances of Level 3 financial assets
Balance at beginning of period	57.6	61.2
Net earnings (loss)	0.0	0.0
Other comp. income (loss)	(16.0)	0.0
Purchases and issuances	0.0	0.0
Sales and settlements	0.0	0.0
Transfer to Level 3	0.0	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	41.6	61.2
Common Stock
Changes in balances of Level 3 financial assets
Balance at beginning of period	3.4	5.9
Net earnings (loss)	0.0	0.2
Other comp. income (loss)	0.0	0.0
Purchases and issuances	0.0	0.0
Sales and settlements	0.0	0.0
Transfer to Level 3	0.0	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	3.4	6.1
Perpetual preferred stocks
Changes in balances of Level 3 financial assets
Balance at beginning of period	54.2	55.3
Net earnings (loss)	0.0	(0.2)
Other comp. income (loss)	(16.0)	0.0
Purchases and issuances	0.0	0.0
Sales and settlements	0.0	0.0
Transfer to Level 3	0.0	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	38.2	55.1
Embedded derivatives
Changes in balances of Level 3 financial liabilities
Balance at beginning of period	3.0	8.4
Net earnings (loss)	(2.3)	(2.3)
Other comp. income (loss)	0.0	0.0
Purchases and issuances	0.0	0.0
Sales and settlements	0.0	0.0
Transfer to Level 3	0.0	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	0.7	6.1
Other
Changes in balances of Level 3 financial liabilities
Balance at beginning of period	4.8	3.5
Net earnings (loss)	(1.1)	(0.8)
Other comp. income (loss)	0.0	0.0
Purchases and issuances	0.0	0.0
Sales and settlements	0.0	0.0
Transfer to Level 3	0.0	0.0
Transfer out of Level 3	0.0	0.0
Balance at end of period	$ 3.7	$ 2.7