Earnings per common share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income (loss) before preference dividends		$ (266,608)	$ 52,539
Preference dividends		(1,171)	(4,907)
Net income (loss) available to common shareholders		$ (267,779)	$ 47,632
Denominator:
Weighted average common shares outstanding - basic and diluted	[1]	19,951,932	22,682,875
Earnings (loss) per common share:
Earnings (loss) per share, basic and diluted		$ (13.42)	$ 2.10
Granted during the period, shares		63,591
Number of restricted share units and common shares that are not vested, shares		48,916
Incremental Common Shares Attributable to Participating Nonvested Shares with Non-forfeitable Dividend Rights		13,727
Antidilutive securities excluded from computation of earnings per share, shares		1,704,691

[1] Warrants held by Arch and HPS were not included in the computation of diluted earnings because the exercise price of the warrants exceeded the market price of the common shares during the period and the exercise of the warrants would have been anti-dilutive. The warrants expired on March 25, 2020. The number of common shares issuable upon exercise of the warrants that was excluded was 1,704,691 common shares.