Schedule IV - Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reinsurance [Line Items]
Life insurance in force, gross	$ 22,197.1	$ 28,750.3	$ 53,831.6
Life insurance in force, ceded to other companies	13,058.0	18,724.6	50,110.5
Life insurance in force, assumed other companies	478.1	516.2	554.1
Life insurance in force, net	$ 9,617.2	$ 10,541.9	$ 4,275.2
Life insurance in force, percentage of amount assumed to net	5.00%	4.90%	13.00%
Premiums, direct amount	$ 15,168.1	$ 14,437.3	$ 11,703.8
Premiums, ceded to other companies	6,960.2	6,634.1	5,700.2
Premiums, assumed from other companies	134.8	216.8	153.3
Net earned premiums	$ 8,342.7	$ 8,020.0	$ 6,156.9
Premiums, percentage of amount assumed to net	1.60%	2.70%	2.50%
Direct policyholder benefits	$ 6,427.5	$ 6,395.6	$ 6,302.9
Benefits, ceded to other companies	4,012.5	3,967.4	4,069.1
Benefits, assumed from other companies	134.3	226.5	108.8
Net policyholder benefits	$ 2,549.3	$ 2,654.7	$ 2,342.6
Benefits, percentage of amount assumed to net	5.30%	8.50%	4.60%
Life insurance
Reinsurance [Line Items]
Premiums, direct amount	$ 299.6	$ 381.3	$ 526.8
Premiums, ceded to other companies	190.3	275.6	402.5
Premiums, assumed from other companies	1.4	2.9	3.8
Net earned premiums	$ 110.7	$ 108.6	$ 128.1
Premiums, percentage of amount assumed to net	1.30%	2.70%	3.00%
Direct policyholder benefits	$ 528.0	$ 555.8	$ 599.9
Benefits, ceded to other companies	240.5	281.9	330.7
Benefits, assumed from other companies	11.0	12.5	12.8
Net policyholder benefits	$ 298.5	$ 286.4	$ 282.0
Benefits, percentage of amount assumed to net	3.70%	4.40%	4.50%
Accident and health insurance
Reinsurance [Line Items]
Premiums, direct amount	$ 623.5	$ 796.5	$ 1,234.2
Premiums, ceded to other companies	446.8	620.5	1,067.8
Premiums, assumed from other companies	0.5	1.6	2.4
Net earned premiums	$ 177.2	$ 177.6	$ 168.8
Premiums, percentage of amount assumed to net	0.30%	0.90%	1.40%
Direct policyholder benefits	$ 438.0	$ 590.5	$ 1,114.4
Benefits, ceded to other companies	387.6	566.3	1,095.8
Benefits, assumed from other companies	(3.9)	0.2	0.4
Net policyholder benefits	$ 46.5	$ 24.4	$ 19.0
Benefits, percentage of amount assumed to net	(8.40%)	0.80%	2.10%
Property and liability insurance
Reinsurance [Line Items]
Premiums, direct amount	$ 14,245.0	$ 13,259.5	$ 9,942.8
Premiums, ceded to other companies	6,323.1	5,738.0	4,229.9
Premiums, assumed from other companies	132.9	212.3	147.1
Net earned premiums	$ 8,054.8	$ 7,733.8	$ 5,860.0
Premiums, percentage of amount assumed to net	1.60%	2.70%	2.50%
Direct policyholder benefits	$ 5,461.5	$ 5,249.3	$ 4,588.6
Benefits, ceded to other companies	3,384.4	3,119.2	2,642.6
Benefits, assumed from other companies	127.2	213.8	95.6
Net policyholder benefits	$ 2,204.3	$ 2,343.9	$ 2,041.6
Benefits, percentage of amount assumed to net	5.80%	9.10%	4.70%