12.STOCK-BASED COMPENSATION

Stock Options

The Company records compensation expense for employee stock options based on the estimated fair value of the options on the date of grant using the Black-Scholes option-pricing model with the assumptions included in the table below. The Company uses a combination of historical data and historical volatility to establish the expected volatility, as well as to estimate the expected option life. The risk-free rate is based on the U.S. Treasury yield curve in effect at the time of grant for the estimated life of the option. The following assumptions were used to estimate the fair value of options granted using the Black-Scholes option-pricing model:



	For the three months ended
	March 31,
	2019	2018

Risk free interest rate	2.47%	2.57%
Dividend yield	1.3%	0.7%
Expected volatility	20%	22%
Expected lives	4.6 years	4.6 years

The following table summarizes the Company’s activities with respect to its stock option plans for the three months ended March 31, 2019 as follows (dollars and shares in thousands, except for per share data):



		Weighted-		Weighted-Average
		Average		Remaining
	Number	Exercise Price		Contractual	Aggregate
	of Shares	Per Share		Life (in years)	Intrinsic Value
Outstanding at December 31, 2018	4,816	$	114.48
Granted	1,035	$	182.30
Exercised	(663)	$	102.44
Forfeited/canceled	(33)	$	127.79
Outstanding at March 31, 2019	5,155	$	129.57	4.6	$	361,222
Exercisable at March 31, 2019	2,550	$	108.03	3.4	$	233,659
Unvested at March 31, 2019	2,605	$	150.67	5.9	$	127,563

The weighted-average per share fair value of options granted during the three months ended March 31, 2019 was $33.84. The total intrinsic value for options exercised during the three months ended March 31, 2019 was $54.1 million.

Restricted Stock Units

The following table summarizes the Company’s restricted stock unit activity for the three months ended March 31, 2019:



		Weighted-Average
	Number of	Grant Date Fair
	Shares	Value per Share
	(in thousands)
Outstanding at December 31, 2018	324	$	128.69
Granted	126	$	182.30
Vested	(123)	$	125.37
Forfeited/canceled	(5)	$	135.24
Outstanding at March 31, 2019	322	$	150.94

During 2018, the Board of Directors adopted a retirement policy applicable to all employees receiving equity as part of their compensation plan. This policy is effective January 1, 2019. Historically, all unvested outstanding equity awards were forfeited upon termination of employment and any options that were vested but unexercised would be forfeited 90 days after the termination of employment. The new retirement policy allows employees that meet certain conditions to vest or continue vesting in outstanding equity awards following retirement and extends the time the employee has to exercise vested and outstanding awards. As a result of this policy, stock compensation expense related to the adoption of the policy resulted in an acceleration of unrecognized stock compensation expense of approximately $11.2 million and $7.3 million in the first and second quarter of 2019, respectively.