SHAREHOLDERS' EQUITY - Preferred shares (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended					2 Months Ended				12 Months Ended
	Jan. 27, 2016	Nov. 30, 2016	May 31, 2013	Apr. 30, 2012	Mar. 31, 2012	Nov. 30, 2005	Feb. 28, 2019	Feb. 28, 2018	Feb. 24, 2017	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 17, 2017
Class of Stock [Line Items]
Aggregate cash payment to acquire preferred shares											$ 0	$ 351,074	$ 51,769
Loss on repurchase of preferred shares											0	0	$ 1,309
Preferred shares, aggregate liquidation value outstanding											$ 775,000	$ 775,000
Series B 7.50% Preferred Shares
Class of Stock [Line Items]
Preferred shares, aggregate liquidation value issued						$ 250,000
Preferred shares, dividend rate											7.50%
Preferred shares, par value per share (in usd per share)											$ 0.0125
Preferred shares, liquidation preference per share (in usd per share)											100
Preferred shares, redemption price per share (in usd per share)											$ 100.00
Preferred shares, purchase price per share (in usd per share)				$ 102.81
Preferred shares, number of shares redeemed (in shares)	28,430			2,471,570
Aggregate cash payment to acquire preferred shares				$ 254,000
Loss on repurchase of preferred shares				$ 9,000
Preferred shares, aggregate redemption price	$ 3,000
Series C 6.875% Preferred Shares
Class of Stock [Line Items]
Preferred shares, aggregate liquidation value issued					$ 400,000
Preferred shares, dividend rate											6.875%
Preferred shares, par value per share (in usd per share)											$ 0.0125
Preferred shares, liquidation preference per share (in usd per share)											25
Preferred shares, redemption price per share (in usd per share)											$ 25
Preferred shares, purchase price per share (in usd per share)		$ 25.67								$ 25.67
Preferred shares, number of shares redeemed (in shares)										1,957,045
Aggregate cash payment to acquire preferred shares										$ 50,000
Loss on repurchase of preferred shares										$ 1,000
Preferred shares, dividend declared per share (in usd per share)												$ 0.4297	$ 1.7188
Preferred shares, dividends paid per share (in usd per share)												0.4297	1.7188
Preferred shares, number of shares outstanding (in shares)														14,042,955
Preferred shares, aggregate liquidation value outstanding														$ 351,000
Series D 5.50% Preferred Shares
Class of Stock [Line Items]
Preferred shares, aggregate liquidation value issued			$ 225,000
Preferred shares, dividend rate											5.50%
Preferred shares, par value per share (in usd per share)											$ 0.0125
Preferred shares, liquidation preference per share (in usd per share)											25
Preferred shares, redemption price per share (in usd per share)											25
Preferred shares, dividend declared per share (in usd per share)											1.3750	1.3750	1.3750
Preferred shares, dividends paid per share (in usd per share)								$ 0.34375			$ 1.0313	1.0313	1.3750
Series E 5.50% Preferred Shares
Class of Stock [Line Items]
Preferred shares, aggregate liquidation value issued		$ 550,000
Preferred shares, dividend rate											5.50%
Preferred shares, par value per share (in usd per share)											$ 0.0125
Preferred shares, liquidation preference per share (in usd per share)											2,500
Preferred shares, redemption price per share (in usd per share)											2,500
Preferred shares, dividend declared per share (in usd per share)											137.50	137.50	$ 34.375
Preferred shares, dividends paid per share (in usd per share)								$ 34.3750	$ 34.375		$ 103.13	$ 103.13
Subsequent Event | Series D 5.50% Preferred Shares
Class of Stock [Line Items]
Preferred shares, dividends paid per share (in usd per share)							$ 0.34375
Subsequent Event | Series E 5.50% Preferred Shares
Class of Stock [Line Items]
Preferred shares, dividend declared per share (in usd per share)							$ 34.375