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Income taxes (Tables)
9 Months Ended
Sep. 30, 2015
Income Tax Disclosure [Abstract]  
Schedule of Components of Income Tax Expense (Benefit)
 
Three months ended
 
Nine months ended
(dollars in thousands)
September 30,
2015
 
September 30,
2014
 
September 30,
2015
 
September 30,
2014
Income tax expense (benefit)
$
8,474

 
$
(2,947
)
 
$
22,975

 
$
2,574

Effective tax rate
31.1
%
 
(25.2
)%
 
24.6
%
 
30.6
%