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Share-based incentives - Summary of the Restricted Common Stock Award Activity (Details) - Restricted Stock
12 Months Ended
Dec. 31, 2015
$ / shares
shares
Shares  
Nonvested balance at January 1, 2015 (in shares) | shares 205,119
Granted (in shares) | shares 118,506
Adjustment for performance-based shares (in shares) | shares 19,610
Vested (in shares) | shares (78,115)
Forfeited (in shares) | shares (12,930)
Nonvested balance at December 31, 2015 (in shares) | shares 252,190
Weighted-Average Grant-Date Fair Value per Share ($)  
Nonvested balance at January 1, 2015 (in usd per share) | $ / shares $ 29.78
Granted (in usd per share) | $ / shares 37.16
Adjustment for performance-based shares (in usd per share) | $ / shares 37.18
Vested (in usd per share) | $ / shares 25.78
Forfeited (in usd per share) | $ / shares 31.43
Nonvested balance at December 31, 2015 (in usd per share) | $ / shares $ 34.88