Net income per common share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income	$ 4,893	$ 294,811	$ 113,940	$ 337,523	$ 262,546	$ (86,781)	$ 413,644	$ 513,288
Weighted-average common shares outstanding
Basic (in shares)	18,455			16,650			17,646	16,750
Diluted (in shares)	18,569			16,809			17,740	16,963
Net income per common share:
Basic (in USD per share)	$ 0.27			$ 20.27			$ 23.44	$ 30.64
Diluted (in USD per share)	$ 0.26			$ 20.08			$ 23.32	$ 30.26
Restricted stock awards
Weighted-average common shares outstanding
Dilutive non-vested stock awards (in shares)	112			159			93	197
Performance share awards
Weighted-average common shares outstanding
Dilutive non-vested stock awards (in shares)	2			0			1	16