EARNINGS PER COMMON SHARE (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended								5 Months Ended	12 Months Ended
	Jan. 15, 2016	Aug. 20, 2015	Aug. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Jan. 15, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings (loss) per common share
Net income (loss)													$ (368,969)	$ 513,368	$ 641,631
Less: Preferred share dividends													46,810	46,597	40,069
Less: Loss on repurchase of preferred shares													0	1,309	0
Net income (loss) available to common shareholders				$ (38,081)	$ (467,740)	$ 85,030	$ 5,014	$ 130,912	$ 176,644	$ 119,491	$ 38,417		$ (415,779)	$ 465,462	$ 601,562
Weighted average common shares outstanding - basic (in shares)													84,108,000	90,772,000	98,609,000
Basic earnings per common share (in dollars per share)				$ (0.46)	$ (5.61)	$ 1.01	$ 0.06	$ 1.50	$ 1.97	$ 1.30	$ 0.41		$ (4.94)	$ 5.13	$ 6.10
Diluted basic earnings (loss) per common share
Net income (loss) available to common shareholders				$ (38,081)	$ (467,740)	$ 85,030	$ 5,014	$ 130,912	$ 176,644	$ 119,491	$ 38,417		$ (415,779)	$ 465,462	$ 601,562
Weighted average common shares outstanding - basic (in shares)													84,108,000	90,772,000	98,609,000
Share-based compensation plans (in shares)													0	775,000	1,020,000
Weighted average common shares outstanding - diluted (in shares)													84,108,000	91,547,000	99,629,000
Diluted earnings per common share (in dollars per share)				$ (0.46)	$ (5.61)	$ 1.01	$ 0.06	$ 1.48	$ 1.96	$ 1.29	$ 0.41		$ (4.94)	$ 5.08	$ 6.04
Weighted average anti-dilutive shares excluded from the dilutive computation (in shares)													702,000	170,000	165,000
Accelerated Share Repurchase Agreement August 17 2015
Accelerated Share Repurchases [Line Items]
Shares repurchased (in shares)	1,358,380	4,149,378	4,149,378									5,507,758		1,358,380	4,149,378