2015 Long-Term Incentive Plan — On June 11, 2015, the Company’s stockholders approved the 2015 Long-Term Incentive Plan (“2015 Plan”) to be used to issue equity awards to company personnel. As of December 31, 2020, 4,549 thousand shares of common stock remained available for issuance under the 2015 Plan. All of the awards issued pursuant to the 2015 Plan expire 10 years from the date of grant.

2012 Non-Employee Directors Compensation Plan — On January 11, 2012, the Company approved the 2012 Non-Employee Directors Compensation Plan (“2012 Directors Plan”) to be used to issue equity grants to its non-employee directors. The Company authorized 600 thousand shares of common stock to be reserved for issuance under the plan. As of December 31, 2020, 524 thousand shares of common stock remained available for issuance under the 2012 Directors Plan. The 2012 Directors Plan will expire after 10 years and is administered by the Company’s Board of Directors.

2012 Long-Term Incentive Plan — On January 11, 2012, the Company approved the 2012 Long-Term Incentive Plan (“2012 Plan”) to be used to issue equity grants to Company personnel. In June 2015, the 2012 Plan was discontinued; however, outstanding awards remain subject to the terms of the 2012 Plan and any shares that are subject to an award that was previously granted under the 2012 Plan and that expire or terminate for any reason prior to exercise will become available for issuance under the 2015 Plan. All of the awards issued pursuant to the 2012 Plan expire 10 years from the date of grant.

2006 Stock Option Plan — Effective May 31, 2006, the Board of Directors of the Company adopted the 2006 Stock Option Plan (the “2006 Plan”) to grant stock options to directors, employees, and certain independent contractors. In January 2012, the 2006 Plan was discontinued; however, outstanding awards remain subject to the terms of the 2006 Plan and any shares that are subject to an option award that was previously granted under the 2006 Plan and that expire or terminate for any reason prior to exercise will become available for issuance under the 2015 Plan. All of the awards issued pursuant to the 2006 Plan expire 10 years from the date of grant.


	Number of Options	Weighted Average Exercise Price		Aggregate Intrinsic Value		Weighted Average Remaining Contractual Term (in years)
Options outstanding as of January 1, 2018	4,902	$	40.91	$	326,064
Options granted	160	$	112.81
Options exercised	(945)	$	36.69
Options forfeited/cancelled	(33)	$	63.28
Options expired	(1)	$	25.72
Options outstanding as of December 31, 2018	4,083	$	44.54	$	291,846
Options granted	132	$	169.13
Options modified	18	$	163.55
Options exercised	(899)	$	41.21
Options forfeited/cancelled	(11)	$	97.83
Options outstanding as of December 31, 2019	3,323	$	50.85	$	536,015
Options granted	158	$	187.76
Options modified	—	$	—
Options exercised	(700)	$	37.79
Options forfeited/cancelled	(9)	$	119.30
Options outstanding as of December 31, 2020	2,772	$	61.71	$	822,152	4.3

Options vested and exercisable as of December 31, 2020	2,388	$	46.99	$	743,582	3.7
Options expected to vest as of December 31, 2020	362	$	152.22	$	74,548	8.2

Expected volatility is based on historical volatility of the Company’s stock price. The expected life represents the period of time that options granted are expected to be outstanding. The risk-free rate is based on the U.S. Treasury yield curve for periods equal to the expected term of the options in effect at the time of grant. The Company has not declared or paid any dividends on its common stock. The Company intends to retain any earnings to fund operations and future growth of its business and, therefore, does not anticipate paying any cash dividends in the foreseeable future.

The weighted-average grant-date fair value of stock options granted during the years ended December 31, 2020, 2019 and 2018 was $68.53, $63.12 and $43.42, respectively. The total intrinsic value of options exercised during the years ended December 31, 2020, 2019 and 2018 was $151.3 million, $121.1 million and $83.3 million, respectively.

The Company recognizes the fair value of each option as compensation expense on a straight-line basis over the requisite service period, which is generally the vesting period. The options are typically scheduled to vest over four years from the time of grant, subject to the terms of the applicable plan and stock option agreement. In general, in the event of a participant’s termination of service for any reason, unvested options are forfeited as of the date of such termination without any payment to the participant. The Company records share-based compensation expense only for those awards that are expected to vest and as such, the Company applies an estimated forfeiture rate at the time of grant and adjusts the forfeiture rate to reflect actual forfeitures quarterly.

The Company grants restricted stock units (“RSUs”) to Company personnel and non-employee directors under the Company’s 2015 Plan (and prior to its approval, under the 2012 Plan) and 2012 Directors Plan, respectively. Prior to 2017, awards to non-employee directors were in the form of restricted stock. In addition, the Company has issued in the past, and may issue in the future, its equity securities to compensate employees of acquired businesses for future services. Equity-based awards granted in connection with acquisitions of businesses may be issued in the form of service-based awards requiring continuing employment with the Company, restricted stock subject to trading restrictions, and performance-based awards, which would vest only if certain specified performance and service conditions are met. The awards issued in connection with acquisitions of businesses are subject to the terms and conditions contained in the applicable award agreements and acquisition documents.


	Equity-Classified Restricted Stock			Equity-Classified Equity-Settled Restricted Stock Units			Liability-Classified Cash-Settled Restricted Stock Units
	Number of Shares	Weighted Average Grant Date Fair Value Per Share		Number of Shares	Weighted Average Grant Date Fair Value Per Share		Number of Shares	Weighted Average Grant Date Fair Value Per Share
Unvested service-based awards outstanding as of January 1, 2018	2	$	54.37	688	$	71.60	315	$	72.50
Awards granted	—	$	—	381	$	115.84	85	$	112.65
Awards modified	—	$	—	(3)	$	80.27	3	$	120.18
Awards vested	(1)	$	47.76	(218)	$	70.10	(92)	$	72.69
Awards forfeited/cancelled	—	$	—	(50)	$	86.97	(8)	$	81.40
Unvested service-based awards outstanding as of December 31, 2018	1	$	63.10	798	$	92.13	303	$	83.99
Awards granted	9	$	167.18	284	$	170.29	56	$	170.13
Awards modified	—	$	—	7	$	170.74	1	$	168.36
Awards vested	—	$	—	(287)	$	87.79	(111)	$	80.51
Awards forfeited/cancelled	—	$	—	(43)	$	114.45	(7)	$	94.77
Unvested service-based awards outstanding as of December 31, 2019	10	$	162.96	759	$	122.48	242	$	105.40
Awards granted	—	$	—	294	$	204.57	60	$	181.77
Awards modified	—	$	—	(1)	$	122.55	—	$	—
Awards vested	(1)	$	63.10	(317)	$	108.87	(122)	$	91.39
Awards forfeited/cancelled	—	$	—	(49)	$	148.11	(5)	$	113.94
Unvested service-based awards outstanding as of December 31, 2020	9	$	167.18	686	$	162.15	175	$	141.16


	For the Years Ended December 31,
	2020		2019		2018
Equity-classified equity-settled
Restricted stock	$	101	$	73	$	142
Restricted stock units	60,042		48,111		24,987
Liability-classified cash-settled
Restricted stock units	22,014		18,449		10,349
Total fair value of vested service-based awards	$	82,157	$	66,633	$	35,478

As of December 31, 2020, $0.8 million of total remaining unrecognized stock-based compensation costs related to service-based equity-classified restricted stock is expected to be recognized over the weighted-average remaining requisite service period of 1.7 years. During the year ended December 31, 2019, the Company issued 9 thousand shares of service-based restricted stock in connection with an acquisition of a business. See Note 2 “Acquisitions” for additional information regarding business acquisitions.

As of December 31, 2020, $76.3 million of total remaining unrecognized stock-based compensation costs related to service-based equity-classified RSUs, net of estimated forfeitures, is expected to be recognized over the weighted-average remaining requisite service period of 2.7 years. During the years ended December 31, 2020 and 2019, in connection with business acquisitions, the Company issued 19 thousand and 22 thousand equity-classified RSUs, respectively. During the first quarter of 2020, in connection with a 2019 acquisition of a business, the Company formally issued 6 thousand equity-classified RSUs. See Note 2 “Acquisitions” for additional information regarding business acquisitions.

As of December 31, 2020, $32.3 million of total remaining unrecognized stock-based compensation costs related to service-based liability-classified RSUs, net of estimated forfeitures, is expected to be recognized over the weighted-average remaining requisite service period of 2.3 years. During the years ended December 31, 2020 and 2019, the Company issued 10 thousand and 7 thousand shares, respectively, of service-based liability-classified cash-settled RSUs in connection with business acquisitions. See Note 2 “Acquisitions” for additional information regarding business acquisitions.


	Equity-Classified Equity-Settled Restricted Stock			Equity-Classified Equity-Settled Restricted Stock Units
	Number of Shares	Weighted Average Grant Date Fair Value Per Share		Number of Shares	Weighted Average Grant Date Fair Value Per Share
Unvested performance-based awards outstanding as of January 1, 2018	—	$	—	—	$	—
Awards granted	—	$	—	45	$	121.75
Awards vested	—	$	—	(8)	$	121.75
Awards forfeited/cancelled	—	$	—	(7)	$	121.75
Unvested performance-based awards outstanding as of December 31, 2018	—	$	—	30	$	121.75
Awards granted	9	$	165.87	—	$	—
Awards modified	—	$	—	(30)	$	121.75
Unvested performance-based awards outstanding as of December 31, 2019	9	$	165.87	—	$	—
Awards granted	—	$	—	31	$	210.44
Awards vested	—	$	—	(10)	$	177.81
Unvested performance-based awards outstanding as of December 31, 2020	9	$	165.87	21	$	227.16

During the year ended December 31, 2020, the Company agreed to issue RSUs at future dates in connection with 2020 acquisitions of businesses. The number of awards to be issued is subject to attainment of specified performance targets in the 2 years after the acquisition dates as well as the Company’s stock price at the time of formal issuance. See Note 2 “Acquisitions” in the consolidated financial statements for additional information regarding business acquisitions. The awards require continued service and vest over 3 years from the dates of acquisition. As of December 31, 2020, taking into consideration the probability of achieving the specified performance goals, $0.5 million of total remaining unrecognized stock-based compensation costs is expected to be recognized over the weighted-average remaining requisite service period of 2 years. If all performance criteria are met, these awards could be worth up to $11.5 million.

During the year ended December 31, 2020, in connection with a 2019 acquisition of a business, the Company formally issued 25 thousand performance-based equity-classified RSUs. During the year ended December 31, 2020, 7 thousand performance-based equity-classified RSUs were granted in connection with 2020 acquisitions of businesses. See Note 2 “Acquisitions” in the consolidated financial statements for additional information regarding business acquisitions. As of December 31, 2020, $3.6 million of total remaining unrecognized stock-based compensation cost related to performance-based equity-classified RSUs is expected to be recognized over the weighted-average remaining requisite service period of 2.8 years.

During the year ended December 31, 2019, the Company issued 9 thousand shares of performance-based equity-classified restricted stock in connection with an acquisition of a business. These awards contain a condition for attainment of specified performance targets in the 12 months after the acquisition date and require continued service through 4.0 years after the acquisition date. As of December 31, 2020, the performance targets were achieved in full and achievement of future vesting is exclusively dependent on continued service. See Note 2 “Acquisitions” for additional information regarding business acquisitions. As of December 31, 2020, $1.0 million of total remaining unrecognized stock-based compensation cost related to performance-based equity-classified restricted stock is expected to be recognized over the weighted-average remaining requisite service period of 2.7 years.

Performance-based equity-classified RSUs were granted during the year ended December 31, 2018 in connection with the acquisition of Continuum with a variable vesting period, subject to satisfaction of the applicable performance conditions with each vesting portion having its own service inception date. Compensation was to be recognized over the vesting period and adjusted each period for the probability of achievement of the performance criteria for each vesting portion separately. During the fourth quarter of 2018, the Company accelerated the recognition of $0.8 million of expense due to vesting of performance-based equity-classified RSUs in accordance with the terms of the award agreement. During the year ended December 31, 2019, the Company and holders of the unvested performance-based equity-classified RSUs mutually agreed to cancel these awards and the Company issued service-based stock option and equity-classified RSU awards with four-year vesting terms to those same recipients.