Share-based payments - Summary of the Restricted Common Stock Award/Unit Activity (Details)	12 Months Ended
	Dec. 31, 2019 $ / shares shares
Restricted Stock Awards
Shares
Nonvested balance at beginning of period (in shares) | shares	120,276
Granted (in shares) | shares	13,702
Adjustment for performance-based shares (in shares) | shares	(40,297)
Vested (in shares) | shares	(33,532)
Forfeited (in shares) | shares	(6,760)
Nonvested balance at end of period (in shares) | shares	53,389
Weighted-Average Grant-Date Fair Value per Share ($)
Nonvested balance at beginning of period (in usd per share) | $ / shares	$ 40.99
Granted (in usd per share) | $ / shares	41.26
Adjustment for performance-based shares (in usd per share) | $ / shares	42.32
Vested (in usd per share) | $ / shares	39.82
Forfeited (in usd per share) | $ / shares	42.50
Nonvested balance at end of period (in usd per share) | $ / shares	$ 40.60
Restricted Stock Unit
Shares
Nonvested balance at beginning of period (in shares) | shares	123,841
Granted (in shares) | shares	122,358
Vested (in shares) | shares	(17,003)
Forfeited (in shares) | shares	(33,179)
Nonvested balance at end of period (in shares) | shares	196,017
Weighted-Average Grant-Date Fair Value per Share ($)
Nonvested balance at beginning of period (in usd per share) | $ / shares	$ 42.67
Granted (in usd per share) | $ / shares	42.02
Vested (in usd per share) | $ / shares	42.14
Forfeited (in usd per share) | $ / shares	43.21
Nonvested balance at end of period (in usd per share) | $ / shares	$ 42.21