Stock-Based Compensation

During 2018, 2017 and 2016, we recorded non-cash stock-based compensation expense totaling $173 million, $158 million and $149 million, which is included as a component of other unallocated, net on our consolidated statements of earnings. The net impact to earnings for the respective years was $137 million, $103 million and $97 million.

As of December 31, 2018, we had $101 million of unrecognized compensation cost related to nonvested awards, which is expected to be recognized over a weighted average period of 1.8 years. We received cash from the exercise of stock options totaling $43 million, $71 million and $106 million during 2018, 2017 and 2016. In addition, our income tax liabilities for 2018, 2017 and 2016 were reduced by $75 million, $203 million and $219 million due to recognized tax benefits on stock-based compensation arrangements.

Stock-Based Compensation Plans

Under plans approved by our stockholders, we are authorized to grant key employees stock-based incentive awards, including options to purchase common stock, stock appreciation rights, RSUs, PSUs or other stock units. The exercise price of options to purchase common stock may not be less than the fair market value of our stock on the date of grant. No award of stock options may become fully vested prior to the third anniversary of the grant and no portion of a stock option grant may become vested in less than one year. The minimum vesting period for restricted stock or stock units payable in stock is three years. Award agreements may provide for shorter or pro-rated vesting periods or vesting following termination of employment in the case of death, disability, divestiture, retirement, change of control or layoff. The maximum term of a stock option or any other award is 10 years.

At December 31, 2018, inclusive of the shares reserved for outstanding stock options, RSUs and PSUs, we had approximately nine million shares reserved for issuance under the plans. At December 31, 2018, approximately five million of the shares reserved for issuance remained available for grant under our stock-based compensation plans. We issue new shares upon the exercise of stock options or when restrictions on RSUs and PSUs have been satisfied.

RSUs

The following table summarizes activity related to nonvested RSUs:



	Number of RSUs (In thousands)		Weighted Average Grant-Date Fair Value Per Share
Nonvested at December 31, 2015	1,236		$	134.87
Granted	679		206.69
Vested	(1,009	)	137.62
Forfeited	(118	)	203.65
Nonvested at December 31, 2016	788		$	183.00
Granted	519		254.58
Vested	(624	)	201.65
Forfeited	(32	)	223.23
Nonvested at December 31, 2017	651		$	220.21
Granted	406		353.99
Vested	(470	)	271.50
Forfeited	(24	)	282.07
Nonvested at December 31, 2018	563		$	271.23

In 2018, we granted certain employees approximately 0.4 million RSUs with a weighted average grant-date fair value of $353.99 per RSU. The grant-date fair value of these RSUs is equal to the closing market price of our common stock on the grant date less a discount to reflect the delay in payment of dividend-equivalent cash payments that are made only upon vesting, which is generally three years from the grant date. We recognize the grant-date fair value of RSUs, less estimated forfeitures, as compensation expense ratably over the requisite service period, which is shorter than the vesting period if the employee is retirement eligible on the date of grant or will become retirement eligible before the end of the vesting period.

Stock Options

We generally recognize compensation cost for stock options ratably over the three-year vesting period. At December 31, 2018 and 2017, there were 1.8 million (weighted average exercise price of $79.76) and 2.2 million (weighted average exercise price of $82.71) stock options outstanding. All of the stock options outstanding are vested as of December 31, 2018 and have a weighted average remaining contractual life of approximately 1.9 years and an aggregate intrinsic value of $326 million. There were 0.4 million (weighted average exercise price of $94.63) stock options exercised during 2018. We have not granted stock options to employees since 2012. The intrinsic value of all stock options exercised was $104 million, $139 million, and $172 million in 2018, 2017 and 2016.

PSUs

In 2018, we granted certain employees PSUs with an aggregate target award of approximately 0.1 million shares of our common stock. The PSUs vest three years from the grant date based on continuous service, with the number of shares earned (0% to 200% of the target award) depending upon the extent to which we achieve certain financial and market performance targets measured over the period from January 1, 2018 through December 31, 2020. About half of the PSUs were valued at $354.60 per PSU in a manner similar to RSUs mentioned above as the financial targets are based on our operating results. We recognize the grant-date fair value of these PSUs, less estimated forfeitures, as compensation expense ratably over the vesting period based on the number of awards expected to vest at each reporting date. The remaining PSUs were valued at $420.75 per PSU using a Monte Carlo model as the performance target is related to our total shareholder return relative to our peer group. We recognize the grant-date fair value of these awards, less estimated forfeitures, as compensation expense ratably over the vesting period.