Schedule IV - Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reinsurance [Line Items]
Life insurance in force, gross	$ 28,750.3	$ 53,831.6	$ 77,852.8
Life insurance in force, ceded to other companies	18,724.6	50,110.5	74,851.8
Life insurance in force, assumed other companies	516.2	554.1	614.8
Life insurance in force, net	$ 10,541.9	$ 4,275.2	$ 3,615.8
Life insurance in force, percentage of amount assumed to net	4.90%	13.00%	17.00%
Premiums, direct amount	$ 14,437.3	$ 11,703.8	$ 9,530.8
Premiums, ceded to other companies	6,634.1	5,700.2	5,280.6
Premiums, assumed from other companies	216.8	153.3	153.9
Net earned premiums	$ 8,020.0	$ 6,156.9	$ 4,404.1
Premiums, percentage of amount assumed to net	2.70%	2.50%	3.50%
Direct policyholder benefits	$ 6,395.6	$ 6,302.9	$ 6,439.5
Benefits, ceded to other companies	3,967.4	4,069.1	4,797.0
Benefits, assumed from other companies	226.5	108.8	228.1
Net policyholder benefits	$ 2,654.7	$ 2,342.6	$ 1,870.6
Benefits, percentage of amount assumed to net	8.50%	4.60%	12.20%
Life insurance
Reinsurance [Line Items]
Premiums, direct amount	$ 381.3	$ 526.8	$ 602.8
Premiums, ceded to other companies	275.6	402.5	465.8
Premiums, assumed from other companies	2.9	3.8	6.1
Net earned premiums	$ 108.6	$ 128.1	$ 143.1
Premiums, percentage of amount assumed to net	2.70%	3.00%	4.30%
Direct policyholder benefits	$ 555.8	$ 599.9	$ 666.1
Benefits, ceded to other companies	281.9	330.7	404.2
Benefits, assumed from other companies	12.5	12.8	14.4
Net policyholder benefits	$ 286.4	$ 282.0	$ 276.3
Benefits, percentage of amount assumed to net	4.40%	4.50%	5.20%
Accident and health insurance
Reinsurance [Line Items]
Premiums, direct amount	$ 796.5	$ 1,234.2	$ 1,424.4
Premiums, ceded to other companies	620.5	1,067.8	1,272.4
Premiums, assumed from other companies	1.6	2.4	4.8
Net earned premiums	$ 177.6	$ 168.8	$ 156.8
Premiums, percentage of amount assumed to net	0.90%	1.40%	3.10%
Direct policyholder benefits	$ 590.5	$ 1,114.4	$ 775.0
Benefits, ceded to other companies	566.3	1,095.8	802.0
Benefits, assumed from other companies	0.2	0.4	0.2
Net policyholder benefits	$ 24.4	$ 19.0	$ (26.8)
Benefits, percentage of amount assumed to net	0.80%	2.10%	(0.70%)
Property and liability insurance
Reinsurance [Line Items]
Premiums, direct amount	$ 13,259.5	$ 9,942.8	$ 7,503.6
Premiums, ceded to other companies	5,738.0	4,229.9	3,542.4
Premiums, assumed from other companies	212.3	147.1	143.0
Net earned premiums	$ 7,733.8	$ 5,860.0	$ 4,104.2
Premiums, percentage of amount assumed to net	2.70%	2.50%	3.50%
Direct policyholder benefits	$ 5,249.3	$ 4,588.6	$ 4,998.4
Benefits, ceded to other companies	3,119.2	2,642.6	3,590.8
Benefits, assumed from other companies	213.8	95.6	213.5
Net policyholder benefits	$ 2,343.9	$ 2,041.6	$ 1,621.1
Benefits, percentage of amount assumed to net	9.10%	4.70%	13.20%