Share-based payments (Tables)

3 Months Ended

Mar. 31, 2020

Share-based payments

Compensation Costs Related to Share-Based Payments

Three months ended March 31,	2020		2019
Stock option compensation expense:
SG&A expenses	$	1,187	$	1,539
Share unit expense:
Equity-classified share units		1,786		2,368
Liability-classified share units		(1,156)		150
Employee share purchase plan - employer contributions		590		553
	$	2,407	$	4,610

Summary of Stock Option Activity

				WA
	Common	WA		remaining		Aggregate
	shares under	exercise		contractual		intrinsic
	option	price		life (in years)		value
Outstanding, December 31, 2019	2,797,189	$	29.05		7.1	$	38,874
Granted	642,380		40.64
Exercised	(247,446)		28.51				3,536
Forfeited	(22,016)		30.11
Expired	(1,201)		21.82
Outstanding, March 31, 2020	3,168,906		31.44		7.6		13,039
Exercisable, March 31, 2020	1,790,002	$	27.68		6.5	$	11,638

Summary of Significant Assumptions Used to Estimate the Fair Value of Stock Options

Three months ended March 31,	2020		2019
Risk free interest rate	0.75	%	2.5	%
Expected dividend yield	2.02	%	2.06	%
Expected lives of the stock options	5	years	5	years
Expected volatility	27.3	%	26.8	%

Summary of Share Unit Activity

	Equity-classified awards						Liability-classified awards
	PSUs			RSUs			DSUs
		WA grant			WA grant				WA grant
		date fair			date fair				date fair
	Number	value		Number	value			Number	value
Outstanding, December 31, 2019	428,724	$	32.89	237,420	$	29.72	$	118,368	$	29.64
Granted	181,883		41.60	23,163		41.11		5,407		40.43
Vested and settled	(156,238)		31.94	(7,539)		36.29		—		—
Forfeited	(2,644)		33.29	(311)		32.82		—		—
Outstanding, March 31, 2020	451,725	$	36.73	252,733	$	30.57	$	123,775	$	30.11