Incentive Plans

Stock-based compensation. For the three months ended March 31, 2019, the Company recorded $29 million of stock-based compensation expense for all plans, as compared to $23 million for the same period in 2018. Stock-based compensation expense for the three months ended March 31, 2019 included $3 million of remaining expense associated with the unvested restricted stock awards and performance units granted to the Company's former CEO, who retired during the period, in accordance with the terms of his award agreements. In addition, the Company accounted for the modification of award agreements with certain other officers that were notified during the three months ended March 31, 2019 of the Company's planned changes to its leadership and organizational structure to align its cost structure with the needs of a Permian Basin-focused company. These other officer modifications provided for the full vesting of all outstanding awards upon each such officers' early retirement and departure from the Company. The modification associated with the full vesting of outstanding awards resulted in a modified fair value that is in excess of what would have vested in accordance with the terms of the original award agreements. The stock-based compensation expense associated with the modified awards will be recognized over the remaining service period of those officers, which is generally expected to be less than 90 days.

As of March 31, 2019, there was $146 million of unrecognized stock-based compensation expense related to unvested share-based compensation plans, including $29 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, 2019 and December 31, 2018, accounts payable – due to affiliates included $3 million and $14 million, respectively, of liabilities attributable to Liability Awards.

Activity for outstanding restricted stock awards and performance units is as follows:



	Three Months Ended March 31, 2019
	Restricted Stock Equity Awards		Restricted Stock Liability Awards		Performance Units
Beginning incentive compensation awards	799,672		201,501		119,169
Awards granted	470,204		125,607		86,483
Awards forfeited	(3,201	)	(2,026	)	—
Awards vested (a)	(391,446	)	(101,219	)	(39,793	)
Ending incentive compensation awards	875,229		223,863		165,859

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(a)	Per the terms of award agreements and elections, the issuance of common stock may be deferred for certain restricted stock equity awards and performance units that vest during the period.