MORTGAGE LOANS - Delinquent Loans (Details) - Commercial mortgage loans $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) loan	Dec. 31, 2017 USD ($) loan
Delinquent loans
Commercial mortgage loans | $	$ 2,423	$ 1,817
Number of delinquent commercial mortgage loans | loan	2	2
30 to 59 Days Delinquent
Delinquent loans
Commercial mortgage loans | $	$ 1,180	$ 1,817
Number of delinquent commercial mortgage loans | loan	1	2
60 to 89 Days Delinquent
Delinquent loans
Commercial mortgage loans | $	$ 0	$ 0
Number of delinquent commercial mortgage loans | loan	0	0
Greater than 90 Days Delinquent
Delinquent loans
Commercial mortgage loans | $	$ 1,243	$ 0
Number of delinquent commercial mortgage loans | loan	1	0