Incentive Plans

Stock-based compensation. For the three months ended March 31, 2018, the Company recorded $23 million of stock-based compensation expense for all plans, as compared to $30 million for the same period in 2017. As of March 31, 2018, there was $163 million of unrecognized stock-based compensation expense related to unvested share-based compensation plans, including $35 million attributable to stock-based awards that are expected to be settled on their vesting date in cash, rather than in equity shares ("Liability Awards"). The unrecognized compensation expense will be recognized on a straight-line basis over the remaining vesting periods of the awards, which is a period of less than three years on a weighted average basis. As of March 31, 2018 and December 31, 2017, accounts payable – due to affiliates included $4 million and $20 million, respectively, of liabilities attributable to Liability Awards.

The following table summarizes the activity that occurred during the three months ended March 31, 2018 for restricted stock awards and performance units issued by Pioneer:



	Restricted Stock Equity Awards		Restricted Stock Liability Awards		Performance Units
Outstanding as of December 31, 2017	916,223		252,735		163,158
Awards granted	369,170		108,129		62,541
Awards forfeited	(11,986	)	(2,652	)	(1,285	)
Awards vested	(395,682	)	(121,776	)	—
Outstanding as of March 31, 2018	877,725		236,436		224,414

As of March 31, 2018 and December 31, 2017, the Company also had 138,493 stock options outstanding and exercisable. There were no stock options exercised during the three months ended March 31, 2018.