STOCKHOLDERS' EQUITY - Share Repurchases (Details) $ / shares in Units, $ in Thousands					1 Months Ended	12 Months Ended
	Dec. 18, 2017 USD ($) $ / shares shares	Jun. 12, 2017 USD ($) $ / shares shares	Sep. 14, 2016 USD ($) $ / shares shares	May 16, 2016 USD ($) $ / shares shares	Jun. 30, 2016 mortgage_loan	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Class of Stock [Line Items]
Aggregate purchase price, repurchase of common stock						$ 886,010	$ 290,134
Number of mortgage loans, proceeds used for share repurchases | mortgage_loan					6
Cash Tender Offer
Class of Stock [Line Items]
Shares repurchase fees and expenses				$ 301
Cash Tender Offer | Common Stock
Class of Stock [Line Items]
Repurchase of common stock (in shares) | shares				10,000,000
Aggregate purchase price, repurchase of common stock				$ 210,000
Share price of stock repurchased (in usd per share) | $ / shares				$ 21
Urban Partners II, LLC | December 2017 Share Repurchase | Common Stock
Class of Stock [Line Items]
Repurchase of common stock (in shares) | shares	14,090,909
Aggregate purchase price, repurchase of common stock	$ 310,000
Share price of stock repurchased (in usd per share) | $ / shares	$ 22
Urban Partners II, LLC | June 2017 Share Repurchase | Common Stock
Class of Stock [Line Items]
Repurchase of common stock (in shares) | shares		26,181,818
Aggregate purchase price, repurchase of common stock		$ 576,000
Share price of stock repurchased (in usd per share) | $ / shares		$ 22
Urban Partners II, LLC | September 2016 Share Repurchase | Common Stock
Class of Stock [Line Items]
Repurchase of common stock (in shares) | shares			3,628,116
Aggregate purchase price, repurchase of common stock			$ 79,819
Share price of stock repurchased (in usd per share) | $ / shares			$ 22
Urban Partners II, LLC | Cash Tender Offer | Common Stock
Class of Stock [Line Items]
Aggregate purchase price, repurchase of common stock				$ 208,140