EARNINGS PER COMMON SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings (loss) per common share
Net income (loss)									$ 43,021	$ (368,969)	$ 513,368
Less: Preferred share dividends									42,625	46,810	46,597
Less: Loss on repurchase of preferred shares									0	0	1,309
Net income (loss) available (attributable) to common shareholders	$ (198,448)	$ 43,439	$ 92,858	$ 62,546	$ (38,081)	$ (467,740)	$ 85,030	$ 5,014	$ 396	$ (415,779)	$ 465,462
Weighted average common shares outstanding (in shares)									83,501	84,108	90,772
Earnings (loss) per common share (in dollars per share)	$ (2.37)	$ 0.52	$ 1.11	$ 0.75	$ (0.46)	$ (5.61)	$ 1.01	$ 0.06	$ 0.00	$ (4.94)	$ 5.13
Earnings (loss) per diluted common share
Net income (loss) available (attributable) to common shareholders	$ (198,448)	$ 43,439	$ 92,858	$ 62,546	$ (38,081)	$ (467,740)	$ 85,030	$ 5,014	$ 396	$ (415,779)	$ 465,462
Weighted average common shares outstanding (in shares)									83,501	84,108	90,772
Share-based compensation plans (in shares)									506	0	775
Weighted average diluted common shares outstanding (in shares)									84,007	84,108	91,547
Diluted earnings (loss) per common share (in dollars per share)	$ (2.37)	$ 0.52	$ 1.11	$ 0.75	$ (0.46)	$ (5.61)	$ 1.01	$ 0.06	$ 0.00	$ (4.94)	$ 5.08
Weighted average anti-dilutive shares excluded from the dilutive computation (in shares)									245	702	170