10. STOCK-BASED COMPENSATION

At December 31, 2018, we had stock-based awards outstanding under stock incentive compensation plans approved by our stockholders. Under these plans, shares have been authorized for issuance to our directors, officers and certain other associates in the form of unvested restricted stock units, performance shares or other awards that are based on the value of our common stock. Shares available for future grants at December 31, 2018 were 5.3 million. The current stock plan will expire in May 2028.

RESTRICTED STOCK UNITS We have awarded restricted stock units (RSUs). Compensation expense associated with RSUs settled in stock is recorded to paid-in-capital ratably over the three-year vesting period.

The following table summarizes activity relating to our RSUs:



			Weighted-Average
	Number of		Grant Date Fair
	Shares/Units		Value per Share/Unit
	(in millions, except per share data)
Outstanding at January 1, 2016	1.7		$	18.19
Granted	0.9		15.41
Vested	(0.7	)	13.23
Canceled	(0.1	)	18.75
Outstanding at December 31, 2016	1.8		$	18.70
Granted	1.3		18.09
Vested	(0.4	)	19.70
Canceled	(0.2	)	16.79
Outstanding at December 31, 2017	2.5		$	18.35
Granted	1.7		14.57
Vested	(0.4	)	24.16
Canceled	(0.3	)	15.84
Outstanding at December 31, 2018	3.5		$	16.00

As of December 31, 2018, unrecognized compensation cost related to unvested RSUs totaled $22.9 million. The weighted average period over which this cost is expected to be recognized is approximately two years. In 2018 and 2017, the total fair market value of RSUs vested was $6.1 million and $8.8 million, respectively.

PERFORMANCE SHARES As of December 31, 2018, we have performance shares (PS) outstanding under our 2012 Omnibus Incentive Plan. We grant performance shares payable in stock to officers which vest in full over a three-year performance period. In 2018, these grants were based equally on a total shareholder return (TSR) measure, and AAM's three-year cumulative free cash flow. In 2017 and 2016, these grants were based equally on a TSR measure and AAM's three-year adjusted earnings before interest, taxes, depreciation and amortization (EBITDA) margin. The TSR metric compares our TSR over the three-year performance period relative to the TSR of our pre-defined competitor peer group. Based on these EBITDA, free cash flow and relative TSR performance metrics, the number of performance shares that will vest will be between 0% and 200% of the grant date amount. Share price appreciation and dividends paid are measured over the performance period to determine TSR. As these awards are settled in stock, the compensation expense is recorded ratably over the vesting period to paid-in-capital.

The following table summarizes activity relating to our performance shares:



			Weighted Average
	Number of		Grant Date Fair
	Shares		Value per Share
EBITDA Awards	(in millions, except per share data)
Outstanding at January 1, 2016	0.3		$	32.27
Granted	0.2		28.04
Vested	—		—
Canceled	—		—
Outstanding at December 31, 2016	0.5		$	30.19
Granted	0.2		39.01
Vested	(0.1	)	27.73
Canceled	—		—
Outstanding at December 31, 2017	0.6		$	33.91
Granted	—		—
Vested	(0.1	)	37.67
Canceled	—		—
Outstanding at December 31, 2018	0.5		$	34.49

TSR Awards
Outstanding at January 1, 2016	0.3		$	25.77
Granted	0.2		13.16
Vested	—		—
Canceled	—		—
Outstanding at December 31, 2016	0.5		$	19.55
Granted	0.2		24.58
Vested	(0.1	)	22.78
Canceled	—		—
Outstanding at December 31, 2017	0.6		$	20.93
Granted	0.3		13.91
Vested	(0.1	)	31.21
Canceled	—		—
Outstanding at December 31, 2018	0.8		$	16.25

Free Cash Flow Awards
Outstanding at January 1, 2018	—		$	—
Granted	0.3		14.28
Vested	—		—
Canceled	—		—
Outstanding at December 31, 2018	0.3		$	14.28

We estimate the fair value of our EBITDA performance shares on the date of grant using our estimated three-year adjusted EBITDA margin, based on AAM's budget and long-range plan assumptions at that time, and adjust quarterly as necessary. We estimate the fair value of our TSR performance shares on the date of grant using the Monte Carlo simulation approach. The Monte Carlo simulation approach utilizes inputs on volatility assumptions, risk free rates, the price of the Company’s and our competitor peer group's common stock and their correlation as of each valuation date. Volatility assumptions are based on historical and implied volatility measurements. We estimate the fair value of our free cash flow performance shares on the date of grant using our estimated three-year cumulative free cash flow, based on AAM's budget and long-range plan assumptions at the time, and adjust quarterly as necessary.

Based on the current fair value, the estimated unrecognized compensation cost related to unvested PS totaled $10.8 million, as of December 31, 2018. The weighted-average period over which this cost is expected to be recognized is approximately two years.