Share-based payments (Tables)

9 Months Ended

Sep. 30, 2020

Share-based payments

Compensation Costs Related to Share-Based Payments

	Three months ended				Nine months ended
	September 30,				September 30,
	2020		2019		2020		2019
Stock option compensation expense:
SG&A expenses	$	1,671	$	1,653	$	4,401	$	4,852
Share unit expense:
Equity-classified share units		4,138		2,851		9,155		8,754
Liability-classified share units		2,123		589		1,938		829
Employee share purchase plan - employer contributions		636		567		1,835		1,692
	$	8,568	$	5,660	$	17,329	$	16,127

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	816,227		41.74
Exercised	(1,430,545)		28.10				31,798
Forfeited	(55,567)		33.53
Expired	(2,064)		20.74
Outstanding, September 30, 2020	2,125,240		34.46		7.8		52,688
Exercisable, September 30, 2020	801,352	$	28.28		6.3	$	24,820

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

Nine months ended September 30,	2020		2019
Risk free interest rate	0.7	%	2.5	%
Expected dividend yield	1.96	%	2.06	%
Expected lives of the stock options	5	years	5	years
Expected volatility	27.9	%	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	300,595		42.00	26,610		40.52		15,427		43.29
Vested and settled	(156,238)		31.94	(7,911)		36.23		—		—
Forfeited	(29,863)		36.39	(48,243)		27.85		—		—
Outstanding, September 30, 2020	543,218	$	38.01	207,876	$	31.29	$	133,795	$	31.21