DERIVATIVES AND HEDGING ACTIVITY - Interest Rate Derivatives (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) instrument item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)
Derivatives
Unrealized holding gain/(loss)	$ 4,806	$ 1,802	$ 3,514
Unsecured Commercial Bank Credit Facility
Derivatives
Total revolving credit facility		$ 1,100,000			$ 2,000,000	$ 2,000,000
Term note due January 2029 | Unsecured Commercial Bank Credit Facility
Derivatives
Total revolving credit facility				$ 300,000
Years to maturity	10 years
Designated as Hedging Instrument | Interest rate swaps
Derivatives
Number of Interest Rate Derivatives Held | item	4
Notional	$ 315,000
Designated as Hedging Instrument | Interest rate swaps subsequently terminated and settled
Derivatives
Number of Interest Rate Derivatives Held | instrument	2
Notional	$ 150,000
Cash received to settle swaps	3,100
Unrealized holding gain/(loss)	3,100
Designated as Hedging Instrument | Interest rate swaps effective from December 2019
Derivatives
Notional	$ 50,000
Designated as Hedging Instrument | Interest rate caps
Derivatives
Number of Interest Rate Derivatives Held | item	1
Notional	$ 65,197
Not Designated as Hedging Instrument | Interest rate caps
Derivatives
Number of Interest Rate Derivatives Held | instrument	1
Notional	$ 19,880