The Company grants stock options and restricted stock units to HMS employees and non-employee directors of the Company under the HMS Holdings Corp. 2019 Omnibus Incentive Plan (the “2019 Omnibus Plan”), as approved by the Company’s shareholders on May 22, 2019. The 2019 Omnibus Plan replaces and supersedes the HMS Holdings Corp. 2016 Omnibus Incentive Plan.

Total stock-based compensation expense in the Company’s unaudited Consolidated Statements of Income related to the Company’s long-term incentive award plans was as follows (in thousands):


	Three Months Ended March 31,
	2020		2019
Cost of services-compensation	$	3,849	$	4,124
Selling, general and administrative	9,661		6,855
Total	$	13,510	$	10,979

For the three months ended March 31, 2020 and 2019, stock-based compensation expense related to stock options was approximately $5.6 million and $4.8 million, respectively.

Presented below is a summary of stock option activity for the three months ended March 31, 2020 (in thousands, except for weighted average exercise price and weighted average remaining contractual terms):


	Number of Options	Weighted Average Exercise Price		Weighted Average- Remaining Contractual Terms	Aggregate Intrinsic Value
Outstanding balance at December 31, 2019	2,411	$	23.43
Granted	1,028	27.34
Exercised	(14)	8.97
Forfeitures	—	—
Expired	—	—
Outstanding balance at March 31, 2020	3,425	$	24.68	7.93	$	12,430
Expected to vest at March 31, 2020	1,363	$	28.72	9.34	$	1,267
Exercisable at March 31, 2020	1,613	$	20.31	6.42	$	10,612

During the three months ended March 31, 2020 and 2019, the Company received proceeds of $0.1 million and $23.1 million, respectively, for the issuance of 13,561 and 1,443,685 shares of common stock upon the exercise of outstanding stock options, respectively. The total intrinsic value of stock options exercised during the three months ended March 31, 2020 and 2019 was $0.2 million and $25.8 million, respectively.

As of March 31, 2020, there was approximately $7.9 million of total unrecognized compensation cost related to stock options outstanding, which is expected to be recognized over a weighted average period of 1.6 years.

The weighted-average grant date fair value per share of the stock options granted during the three months ended March 31, 2020 and 2019 was $8.91 and $13.82, respectively. HMS estimated the fair value of each stock option grant on the date of grant using a Black-Scholes option pricing model and weighted–average assumptions set forth in the following table:


	Three Months Ended March 31,
	2020		2019
Expected dividend yield	0	%	0	%
Risk-free interest rate	1.2	%	2.5	%
Expected volatility	41.1	%	40.9	%
Expected life (years)	6.3		6.4

The total tax benefits recognized on stock-based compensation related to stock options for the three months ended March 31, 2020 and 2019 was $2.6 million and $11.3 million, respectively.

For the three months ended March 31, 2020 and 2019, stock-based compensation expense related to restricted stock units was approximately $7.9 million and $6.1 million, respectively.

Presented below is a summary of restricted stock units activity for the three months ended March 31, 2020 (in thousands, except for weighted average grant date fair value per unit):


	Number of Units	Weighted Average Grant Date Fair Value per Unit
Outstanding balance at December 31, 2019	1,239	$	21.37
Granted	693	23.78
Vesting of restricted stock units, net of units withheld for taxes	(293)	22.67
Units withheld for taxes	(141)	22.67
Forfeitures	(1)	24.36
Outstanding balance at March 31, 2020	1,497	$	21.60

As of March 31, 2020, 1,224,176 restricted stock units remained unvested and there was approximately $18.3 million of unrecognized compensation cost related to restricted stock units, which is expected to be recognized over a weighted average vesting period of 1.6 years.