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Earnings per share (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Basic earnings per share [Abstract]                      
Basic earnings per share (in dollars per share) $ 0.28 $ 0.20 $ 0.41 $ 0.30 $ 0.33 $ 0.24 $ 0.00 $ 0.00      
Diluted earnings per share [Abstract]                      
Diluted earnings per share (in dollars per share) $ 0.27 $ 0.20 $ 0.40 $ 0.29 $ 0.32 $ 0.24 $ 0.00 $ 0.00      
Class A Common Stock [Member]                      
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]                      
Earnings allocable                 $ 0 $ 13,625 $ 21,092
Basic earnings per share [Abstract]                      
Basic earnings per share (in dollars per share)                 $ 0.00 $ 0.98 $ 1.53
Diluted earnings per share [Abstract]                      
Diluted earnings per share (in dollars per share)                 $ 0.00 $ 0.98 $ 1.53
Basic weighted average shares outstanding [Abstract]                      
Basic weighted average shares outstanding (in shares)                 0 13,844,000 13,796,000
Diluted weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - diluted (in shares)                 0 13,844,000 13,796,000
Common stock, dividend rate                 12.00%    
Percentage of residual proceeds 75.00%               75.00%    
Antidilutive securities (in shares)                   0 0
Class B Common Stock [Member]                      
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]                      
Earnings allocable                 $ 0 $ 760 $ 1,181
Basic earnings per share [Abstract]                      
Basic earnings per share (in dollars per share)                 $ 0.00 $ 0.48 $ 0.84
Diluted earnings per share [Abstract]                      
Diluted earnings per share (in dollars per share)                 $ 0.00 $ 0.46 $ 0.81
Basic weighted average shares outstanding [Abstract]                      
Basic weighted average shares outstanding (in shares)                 0 1,574,000 1,413,000
Dilutive effect of shares issued under stock compensation arrangements [Abstract]                      
Dilutive effect of shares issued under stock compensation arrangements (in shares)                 0 70,000 39,000
Diluted weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - diluted (in shares)                 0 1,644,000 1,452,000
Percentage of residual proceeds 25.00%               25.00%    
Common Stock [Member]                      
Reconciliation of numerator and denominator of the basic and diluted earnings per share computations [Abstract]                      
Earnings allocable                 $ 24,901 $ 11,782 $ 0
Basic earnings per share [Abstract]                      
Basic earnings per share (in dollars per share)                 $ 1.19 $ 0.57 $ 0.00
Diluted earnings per share [Abstract]                      
Diluted earnings per share (in dollars per share)                 $ 1.16 $ 0.56 $ 0.00
Basic weighted average shares outstanding [Abstract]                      
Basic weighted average shares outstanding (in shares)                 20,992,000 20,841,000 0
Dilutive effect of shares issued under stock compensation arrangements [Abstract]                      
Dilutive effect of shares issued under stock compensation arrangements (in shares)                 506,000 232,000 0
Diluted weighted average shares outstanding [Abstract]                      
Weighted average shares outstanding - diluted (in shares)                 21,498,000 21,073,000 0
Restricted Stock [Member] | Class B Common Stock [Member]                      
Diluted weighted average shares outstanding [Abstract]                      
Antidilutive securities (in shares)                   0 0
Stock Option [Member]                      
Diluted weighted average shares outstanding [Abstract]                      
Antidilutive securities (in shares)                 0