Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Numerator for basic and diluted earnings per share
Net income (loss)	$ 4,917	$ (103,540)
Denominator for basic earnings per share
Weighted average common shares - Basic for Class A and Class B (in shares)	45,830,286	45,456,214
Denominator for diluted earnings per share (1)
Weighted average common shares - Diluted for Class A and Class B (in shares)	46,194,495	45,456,214
Earnings per share
Basic for Class A and Class B (in dollars per share)	$ 0.11	$ (2.28)
Diluted for Class A and Class B (in dollars per share)	$ 0.11	$ (2.28)
Unvested restricted stock units - stock settled
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)	606,946	0
Unvested restricted stock awards
Earnings per share
Participating securities excluded from earnings per share calculations (in shares)	1,439,810	886,482

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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