CAPITAL STOCK - Completed public offerings ofcommon stock (Details) - USD ($)	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Issued During Period, Value, New Issues										$ 197,607,000	$ 118,328,000	$ 83,275,000
At the Market Offering Program [Member]
Book Value Per Share	$ 8.71	$ 9.15	$ 9.23	$ 9.75	$ 10.1	$ 11.21	$ 10.86	$ 11.01	$ 9.15	$ 8.71	$ 10.1
Weighted Average Price Received Per Share	$ 9.81	$ 0	$ 10.17	$ 10.13	$ 10.79	$ 10.8	$ 10.48		$ 0	$ 9.81	$ 10.79
Stock Issued During Period Shares New Issues	7,746,052	0	11,012,836	1,286,196	6,707,101	3,818,802	646,753	0	20,045,084		11,172,656
Stock Issued During Period, Value, New Issues	$ 74,750,000	$ 0	$ 110,065,000	$ 12,792,000	$ 71,212,000	$ 40,525,000	$ 6,591,000	$ 0	$ 197,607,000		$ 118,328,000