Schedule IV - Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Life insurance in force
Life insurance in force gross amount	$ 486,887.5	$ 437,977.4	$ 390,603.3
Life insurance in force ceded to other companies	287,538.8	242,777.7	184,588.9
Life insurance in force assumed from other companies	1,059.5	1,087.8	1,187.1
Life insurance in force net amount	$ 200,408.2	$ 196,287.5	$ 207,201.5
Life insurance in force, percentage of amount assumed to net (as a percent)	0.50%	0.60%	0.60%
Premiums:
Premiums gross amount	$ 6,699.3	$ 5,753.8	$ 5,710.8
Premiums ceded to other companies	483.8	456.4	402.4
Premiums assumed from other companies	1.9	1.7	1.9
Premiums net amount	6,217.4	5,299.1	5,310.3
Life insurance and annuities
Premiums:
Premiums gross amount	4,945.7	4,137.1	4,208.7
Premiums ceded to other companies	320.7	285.2	255.3
Premiums assumed from other companies	1.9	1.7	1.9
Premiums net amount	4,626.9	3,853.6	3,955.3
Accident and health insurance premiums
Premiums:
Premiums gross amount	1,753.6	1,616.7	1,502.1
Premiums ceded to other companies	163.1	171.2	147.1
Premiums net amount	$ 1,590.5	$ 1,445.5	$ 1,355.0