REINSURANCE - Receivables (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Reinsurance
Reinsurance Recoverables	$ 4,800,891	$ 5,070,185
Concentrated credit risk | Security Life of Denver Insurance Company | A
Reinsurance
Reinsurance Recoverables	740,800	762,200
Concentrated credit risk | Swiss Re Life & Health America, Inc. | A+
Reinsurance
Reinsurance Recoverables	614,800	682,600
Concentrated credit risk | Lincoln National Life Insurance Co. | A+
Reinsurance
Reinsurance Recoverables	489,100	530,900
Concentrated credit risk | Transamerica Life Insurance Co. | A+
Reinsurance
Reinsurance Recoverables	335,600	367,800
Concentrated credit risk | SCOR Global Life | A+
Reinsurance
Reinsurance Recoverables	331,800
Concentrated credit risk | SCOR Global Life | A
Reinsurance
Reinsurance Recoverables		354,800
Concentrated credit risk | RGA Reinsurance Company | A+
Reinsurance
Reinsurance Recoverables	278,300	269,000
Concentrated credit risk | American United Life Insurance Company | A+
Reinsurance
Reinsurance Recoverables	266,700	285,600
Concentrated credit risk | Scottish Re (U.S.) Inc.
Reinsurance
Reinsurance Recoverables	249,500	232,800
Concentrated credit risk | Centre Reinsurance (Bermuda) Ltd
Reinsurance
Reinsurance Recoverables	212,200	243,600
Concentrated credit risk | Employers Reassurance Corporation | A-
Reinsurance
Reinsurance Recoverables	$ 193,900	$ 201,700