Earnings per common share - Computation of basic and diluted earnings per common share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income (loss) before preference dividends		$ 189,937	$ 18,733	$ (76,671)	$ 71,272
Preference dividends		(1,109)	(4,908)	(2,280)	(9,815)
Net income (loss) available to common shareholders		$ 188,828	$ 13,825	$ (78,951)	$ 61,457
Denominator:
Weighted average common shares outstanding - basic		19,863,048	22,740,762	19,907,490	22,711,833
Weighted average non-vested restricted share units	[1],[2]	0	6,271	0	3,136
Weighted average common shares outstanding - diluted	[3]	19,863,048	22,747,033	19,907,490	22,714,969
Earnings (loss) per common share:
Earnings (loss) per share, basic and diluted		$ 9.51	$ 0.61	$ (3.97)	$ 2.71

[1] During the three months ended June 30, 2020, the Company did not grant any restricted share units or common shares. During the six months ended June 30, 2020 and June 30, 2019, the Company granted an aggregate of 63,591 and 165,287, respectively, of restricted share units and common shares to certain employees and directors. Of the total restricted share units and common shares granted, 103,820 are non-vested as of June 30, 2020. The weighted average non-vested restricted share units of 6,864 are excluded from the calculation of diluted weighted average common shares outstanding for the six months ended June 30, 2020, due to a net loss reported. Refer to Note 17, “Share transactions” for further details.

[2] During the three months ended June 30, 2020, the Company issued 100,958 common shares, relating to restricted share units granted in the second quarter of 2019. Of these shares, 27,456 common shares vested during the three months ended June 30, 2020.

[3] 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.