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Earnings per share (Details) - USD ($)
$ / shares in Units, $ 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
Basic earnings per share [Abstract]                      
Earnings per share - basic $ 0.21 $ 0.57 $ 0.48 $ 0.35 $ 0.28 $ 0.20 $ 0.41 $ 0.30      
Diluted earnings per share [Abstract]                      
Earnings per share - diluted $ 0.20 $ 0.55 $ 0.47 $ 0.34 $ 0.27 $ 0.20 $ 0.40 $ 0.29      
Class A Common Stock [Member]                      
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]                      
Earnings allocable                 $ 0 $ 0 $ 13,625
Basic earnings per share [Abstract]                      
Earnings per share - basic                 $ 0.00 $ 0.00 $ 0.98
Diluted earnings per share [Abstract]                      
Earnings per share - diluted                 $ 0.00 $ 0.00 $ 0.98
Basic weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - basic                 0 0 13,844,000
Diluted weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - diluted (in shares)                 0 0 13,844,000
Common stock, dividend rate                     12.00%
Percentage of residual proceeds                     75.00%
Antidilutive securities (in shares)                     0
Class B Common Stock [Member]                      
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]                      
Earnings allocable                 $ 0 $ 0 $ 760
Basic earnings per share [Abstract]                      
Earnings per share - basic                 $ 0.00 $ 0.00 $ 0.48
Diluted earnings per share [Abstract]                      
Earnings per share - diluted                 $ 0.00 $ 0.00 $ 0.46
Basic weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - basic                 0 0 1,574,000
Dilutive effect of shares issued under stock compensation arrangements [Abstract]                      
Dilutive effect of shares issued under stock compensation arrangements (in shares)                 0 0 70,000
Diluted weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - diluted (in shares)                 0 0 1,644,000
Percentage of residual proceeds                     25.00%
Common Stock [Member]                      
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]                      
Earnings allocable                 $ 33,787 $ 24,901 $ 11,782
Basic earnings per share [Abstract]                      
Earnings per share - basic                 $ 1.60 $ 1.19 $ 0.57
Diluted earnings per share [Abstract]                      
Earnings per share - diluted                 $ 1.56 $ 1.16 $ 0.56
Basic weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - basic                 21,090,000 20,992,000 20,841,000
Dilutive effect of shares issued under stock compensation arrangements [Abstract]                      
Dilutive effect of shares issued under stock compensation arrangements (in shares)                 595,000 506,000 232,000
Diluted weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - diluted (in shares)                 21,685,000 21,498,000 21,073,000
Antidilutive securities (in shares)                 86,000 0  
Restricted Stock [Member] | Class B Common Stock [Member]                      
Diluted weighted average shares outstanding [Abstract]                      
Antidilutive securities (in shares)                     0