MORTGAGE LOANS - Loan Portfolio (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
MORTGAGE LOANS
Mortgage loan holdings	$ 6,472,861	$ 6,132,125
Period for exercise of call options or interest rate reset options	12 years
Amount that would become due in remainder of 2017, if loans are called at their next call dates	$ 60,900
Amount that would become due in 2018 through 2022, if loans are called at their next call dates	1,010,200
Amount that would become due in 2023 through 2027, if loans are called at their next call dates	125,700
Amount that would become due after 2027, if loans are called at their next call dates	$ 10,000