SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		12 Months Ended
	Apr. 30, 2018	Apr. 02, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Subsequent events
Repayments of debt			$ 162,034	$ 38,652
6.45% Series D Preferred Stock
Subsequent events
Preferred stock, dividend rate			6.45%		6.45%
6.25% Series E Preferred Stock
Subsequent events
Preferred stock, dividend rate			6.25%		6.25%
Subsequent Event
Subsequent events
Cash dividends declared, common stock (in dollars per share)	$ 0.18
Subsequent Event | 6.45% Series D Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)	0.403125
Subsequent Event | 6.25% Series E Preferred Stock
Subsequent events
Cash dividends declared, preferred stock (in dollars per share)	$ 0.390625
Western Alliance Bank | Mortgage loans | Subsequent Event
Subsequent events
Repayments of debt		$ 23,900
Debt instrument, effective interest rate		5.39%