Share-based payments (Tables)

6 Months Ended

Jun. 30, 2020

Share-based payments

Compensation Costs Related to Share-Based Payments

	Three months ended				Six months ended
	June 30,				June 30,
	2020		2019		2020		2019
Stock option compensation expense:
SG&A expenses	$	1,543	$	1,660	$	2,730	$	3,199
Share unit expense (recovery):
Equity-classified share units		3,231		3,535		5,017		5,903
Liability-classified share units		971		90		(185)		240
Employee share purchase plan - employer contributions		610		572		1,199		1,125
	$	6,355	$	5,857	$	8,761	$	10,467

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	772,074		40.84
Exercised	(679,277)		28.60				9,600
Forfeited	(28,651)		30.55
Expired	(1,201)		21.82
Outstanding, June 30, 2020	2,860,134		32.33		7.6		24,496
Exercisable, June 30, 2020	1,397,769	$	27.41		6.1	$	18,785

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

Six months ended June 30,	2020		2019
Risk free interest rate	0.70	%	2.5	%
Expected dividend yield	1.98	%	2.06	%
Expected lives of the stock options	5	years	5	years
Expected volatility	27.8	%	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	286,773		41.79	25,832		40.80		10,772		40.78
Vested and settled	(156,238)		31.94	(7,724)		36.23		—		—
Forfeited	(9,129)		33.79	(42,817)		27.20		—		—
Outstanding, June 30, 2020	550,130	$	37.78	212,711	$	31.34	$	129,140	$	30.57