Share-based payments (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of the restricted common stock award activity

A summary of the restricted common stock award activity during the year ended December 31, 2017 is presented below:

	Shares		Weighted-Average Grant-Date Fair Value per Share ($)
Nonvested balance at January 1, 2017	206,327		37.18
Granted	132,957		42.59
Adjustment for performance-based shares	(59,348	)	42.78
Vested	(59,260	)	36.98
Forfeited	(76,712	)	37.28
Nonvested balance at December 31, 2017	143,964		39.89