STOCKHOLDERS’ EQUITY

Equity-Based Incentive Plans

Our 2009 Equity Incentive Plan (2009 Plan) provides for the grant of stock awards, including stock options, restricted stock units (time-based and performance-based), restricted stock awards (time-based and performance-based) and other equity awards. The number of shares available for grant under this 2009 Plan as of June 30, 2018 was approximately 12 million.

The following table summarizes stock option activity under our 2009 Plan for the six months ended June 30, 2018:



	Number of Shares
Balance at December 31, 2017	1,296,863
Exercised	(469,894	)
Forfeited	(17,846	)
Balance at June 30, 2018	809,123
Exercisable at June 30, 2018	766,294

The aggregate intrinsic value of stock options outstanding was $35 million and $41 million as of June 30, 2018 and December 31, 2017, respectively.

In March 2018, the Equity Award Committee of the Compensation Committee granted awards of time-based restricted stock (RSAs) and performance-based restricted stock (PRSAs) to the Company's named executive officers. A recipient of RSAs owns the underlying shares of common stock upon grant and some of the benefits of ownership, such as voting and dividend rights, but the recipient may not sell those shares and realize any value on a sale, until all time-based and performance-based restrictions have been satisfied or lapsed.

For non-new hire equity awards our RSAs and restricted stock units (RSUs) are eligible to vest in equal installments on a quarterly basis over four years, subject to continued employment through the applicable vesting dates. The PRSAs are earned based on the extent to which we meet or exceed certain annual growth rate percentages. Our PRSAs granted in 2018 are designed with a single-year performance period subject to subsequent multi-year vesting requirements. Fifty percent of the shares earned (if any) during performance period (January 1, 2018 to December 31, 2018) will vest on December 31, 2019 and the remaining shares earned (if any) will vest on December 31, 2020. The following tables summarize RSU, performance-based restricted stock unit (PSU), RSA, and PRSA activity under our 2009 Plan for the six months ended June 30, 2018:



	RSUs				PSUs
	Number of Units		Weighted-Average Grant Date Fair Value		Number of Units		Weighted-Average Grant Date Fair Value
Nonvested at December 31, 2017	2,249,661		$	24.83	453,674		$	30.72
Granted	585,484		47.36		23,842		47.61
Vested	(545,292	)	25.09		(82,066	)	33.51
Forfeited	(200,961	)	26.76		(65,557	)	31.60
Nonvested at June 30, 2018	2,088,892		$	30.89	329,893		$	31.08



	RSAs				PRSAs
	Number of Units		Weighted-Average Grant Date Fair Value		Number of Units	Weighted-Average Grant Date Fair Value
Nonvested at December 31, 2017	—		$	—	—	$	—
Granted	116,559		49.12		256,224	48.98
Vested	(4,560	)	47.61		—	—
Nonvested at June 30, 2018	111,999		$	49.81	256,224	$	48.98

Stock-Based Compensation

Stock-based compensation expense is measured based on the fair value of the stock option on the grant date and recognized over the requisite service period for each separately vesting portion of the stock option award. Stock-based compensation expense and other disclosures for stock-based awards made to our employees pursuant to the equity plans was as follows:



	Three Months Ended June 30,				Six Months Ended June 30,
(in millions)	2018		2017		2018		2017
Cost of providing services	$	2	$	2	$	4	$	4
Sales and marketing	2		2		4		3
General and administrative	5		3		9		5
Systems development and programming costs	1		1		2		2
Total stock-based compensation expense	$	10	$	8	$	19	$	14
Income tax benefit related to stock-based compensation expense	$	3	$	3	$	5	$	5
Tax benefit realized from stock options exercised and similar awards	$	7	$	10	$	13	$	16

Stock Repurchases

The board of directors authorizes repurchases through an ongoing program initiated in May 2014. During the six months ended June 30, 2018, we repurchased 594,799 shares of common stock for approximately $30 million. As of June 30, 2018, approximately $106 million remained available for further repurchases of our common stock under our ongoing stock repurchase program under all authorizations from our board of directors.