Share-based payments consist of the following compensation costs:




Year ended December 31,	2020		2019		2018
SG&A expenses:
Stock option compensation expense	$	5,853	$	4,697	$	7,895
Equity-classified share units		9,897		8,047		11,256
Liability-classified share units		3,635		1,380		1,764
Employee share purchase plan - employer contributions		2,497		2,281		2,174
		21,882		16,405		23,089
Acquisition-related costs:
Stock option compensation expense		—		—		357
Share-based continuing employment costs		802		—		—
		802		—		357
	$	22,684	$	16,405	$	23,446

Share unit activity for the years ended December 31, 2020, 2019, and 2018 is presented below:


	Equity-classified awards												Liability-classified awards
	PSUs			RSUs			PSUs⁽¹⁾			RSUs			DSUs
		WA grant			WA grant			WA grant			WA grant				WA grant
		date fair			date fair			date fair			date fair				date fair
	Number	value		Number	value		Number	value		Number	value			Number	value
Outstanding, December 31, 2017	434,248	$	27.83	125,152	$	26.93	259,241	$	26.38	4,666	$	26.42		93,487	$	26.32
Granted	240,803		35.63	91,251		32.10	—		—	66		34.70		26,553		33.42
Transferred to (from) equity awards on modification	257,659		31.30	—		—	(257,659)		26.38	—		—		—		—
Vested and settled	(212,263)		33.78	—		—	—		—	(4,732)		26.54		(6,605)		23.16
Forfeited	(50,159)		31.41	(8,417)		32.00	(1,582)		26.45	—		—		—		—

Outstanding, December 31, 2018	670,288	$	31.46	207,986	$	28.99	—	$	—	—	$	—		113,435	$	28.16
Granted	170,208		36.13	36,350		35.40	—		—	—		—		24,810		35.99
Transferred to equity awards on modification	—		—	—		—	—		—	—		—		—		—
Vested and settled	(251,883)		30.33	(265)		31.98	—		—	—		—		(19,877)		27.45
Forfeited	(159,889)		33.21	(6,651)		34.27	—		—	—		—		—		—
Outstanding, December 31, 2019	428,724	$	32.89	237,420	$	29.72	—	$	—	—	$	—	$	118,368	$	29.64
Granted	303,829		42.09	45,781		51.36	—		—	—		—		19,146		47.01
Vested and settled	(156,238)		31.94	(98,542)		27.68	—		—	—		—		—		—
Forfeited	(33,639)		36.60	(49,722)		28.14	—		—	—		—		—		—
Outstanding, December 31, 2020	542,676	$	38.09	134,937	$	39.14	—	$	—	—	$	—	$	137,514	$	32.06

(1)	Liability-classified PSUs include PSUs awarded under the employee PSU plan and the previous 2013 PSU plan, in place prior to 2015, are cash-settled and not subject to market vesting conditions.