MORTGAGE LOANS - Impaired Loans (Details) - Commercial mortgage loans - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Recorded Investment
With no related allowance recorded	$ 730	$ 0
With an allowance recorded	14,373	5,684
Unpaid Principal Balance
With no related allowance recorded	722	0
With an allowance recorded	14,259	5,309
Related Allowance
With an allowance recorded	4,056	1,296
Average Recorded Investment
With no related allowance recorded	243	0
With an allowance recorded	3,593	1,895
Interest Income Recognized
With no related allowance recorded	20	0
With an allowance recorded	540	267
Cash Basis Interest Income
With no related allowance recorded	28	0
With an allowance recorded	$ 536	$ 293

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