We have a stock award and incentive program to attract and retain officers and independent directors. As of December 31, 2019, approximately 3.9 million shares were available for issuance under our Amended and Restated 2015 Stock Award and Incentive Plan, or the 2015 Plan. The total number of shares available for issuance under this plan may increase due to any forfeiture, cancellation, exchange, surrender, termination or expiration of an award outstanding under our 2007 Stock Award and Incentive Plan. Awards under the 2015 Plan may be in the form of incentive stock options, non-qualified stock options and restricted stock, or other types of awards as authorized under the plan.

Our plans are administered by the Compensation and Human Resources Committee of Aimco’s Board of Directors. In the case of stock options, the exercise price of the options granted may not be less than the fair market value of a share of Common Stock at the date of grant.

Total compensation cost recognized for share-based awards was as follows for the years ended December 31, 2019, 2018, and 2017 (in thousands):

	2019		2018		2017
Share-based compensation expense (1)	$	8,146	$	8,550	$	7,877
Capitalized share-based compensation (2)		962		1,215		1,374
Total share-based compensation (3)	$	9,108	$	9,765	$	9,251

As of December 31, 2019, total unvested compensation cost not yet recognized was $10.3 million. We expect to recognize this compensation over a weighted-average period of approximately 1.6 years.

We grant stock options and restricted stock awards that are subject to time-based vesting and require continuous employment, typically over a period of four years from the grant date, and we refer to these awards as Time-Based Stock Options and Time-Based Restricted Stock, respectively. We also grant stock options, restricted stock awards, and two forms of long-term incentive partnership units, or LTIP units, that vest conditioned on Aimco’s total shareholder return, or TSR, relative to the NAREIT Equity Apartment Index (60% weighting) and the MSCI US REIT Index (40% weighting) over a forward-looking performance period of three years. We refer to these awards as TSR Stock Options, TSR Restricted Stock, TSR LTIP I units, and TSR LTIP II units. Vested LTIP II units may be converted at the holders’ option to LTIP Units for a strike price over a term of 10 years. Earned TSR-based awards, if any, will vest 50% on each of the third anniversary and fourth anniversary of the grant date, based on continued employment. Our Time-Based Stock Options and TSR Stock Options expire generally 10 years from the date of grant.

We recognize compensation cost associated with Time-Based awards ratably over the requisite service periods, which are typically four years. We recognize compensation cost related to the TSR-based awards, which have graded vesting periods, over the requisite service period for each separate vesting tranche of the award, commencing on the grant date. The value of the TSR-based awards take into consideration the probability that the market condition will be achieved; therefore previously recorded compensation cost is not adjusted in the event that the market condition is not achieved and awards do not vest.

We had Time-Based Stock Options, Time-Based Restricted Stock, TSR Stock Options, TSR Restricted Stock, TSR LTIP I units and TSR LTIP II units outstanding as of December 31, 2019.

The following table summarizes activity for our outstanding stock options, for the years ended December 31, 2019, 2018, and 2017 (options in thousands):

	2019					2018					2017
	Number of Options			Weighted-Average Exercise Price		Number of Options			Weighted-Average Exercise Price		Number of Options			Weighted-Average Exercise Price
Outstanding at beginning of year		646		$	40.12		648		$	40.08		675		$	29.55
Granted		—			—		—			—		184			44.07
Exercised		(5	)		8.92		(2	)		28.33		(211	)		9.90
Forfeited		—			—		—			—		—			—
Outstanding at end of year		641		$	40.30		646		$	40.12		648		$	40.08
Exercisable at end of year		458		$	38.78		186		$	38.18		128		$	37.59

The intrinsic value of a stock option represents the amount by which the current price of the underlying stock exceeds the exercise price of the option. As of December 31, 2019, stock options outstanding had an aggregate intrinsic value of $7.3 million and a weighted-average remaining contractual term of 6 years. Stock options exercisable as of December 31, 2019, had an aggregate intrinsic value of $5.9 million and a weighted-average remaining contractual term of 5.5 years. The intrinsic value of stock options exercised during the years ended December 31, 2019, 2018, and 2017, was $0.1 million, $0.0 million and $7.1 million, respectively.

During 2017, we granted TSR Stock Options. The weighted-average grant date fair value of stock options granted during the year ended 2017 was $11.39 per option.

The following table summarizes activity for Time-Based Restricted Stock awards for the years ended December 31, 2019, 2018, and 2017 (shares in thousands):

	2019					2018					2017
	Number of Shares			Weighted-Average Grant-Date Fair Value		Number of Shares			Weighted-Average Grant-Date Fair Value		Number of Shares			Weighted-Average Grant-Date Fair Value
Unvested at beginning of year		121		$	40.82		155		$	37.63		241		$	33.61
Granted		48			47.71		49			40.01		44			44.07
Vested		(75	)		42.76		(83	)		34.42		(130	)		32.35
Forfeited		(2	)		38.80		—			—		—			—
Unvested at end of year		92		$	42.86		121		$	40.82		155		$	37.63

The aggregate fair value of Time-Based Restricted Stock awards and TSR Restricted Stock awards that vested during the years ended December 31, 2019, 2018, and 2017 was $13.7 million, $8.4 million and $6.0 million, respectively.

The following table summarizes activity for TSR Restricted Stock awards for the years ended December 31, 2019, 2018, and 2017 (shares in thousands):

	2019					2018					2017
	Number of Shares			Weighted-Average Grant-Date Fair Value		Number of Shares			Weighted-Average Grant-Date Fair Value		Number of Shares		Weighted-Average Grant-Date Fair Value
Unvested at beginning of year		171		$	41.65		246		$	40.70		208	$	39.66
Granted (1)		39			54.73		44			41.71		38		46.39
Change in awards (2)		216			39.67		—			—		—		—
Vested		(213	)		39.67		(119	)		39.72		—		—
Unvested at end of year		213		$	43.99		171		$	41.65		246	$	40.70

The following table summarizes activity for TSR LTIP I units for the years ended December 31, 2019, 2018, and 2017 (units in thousands):

	2019				2018				2017
	Number of Units		Weighted-Average Grant-Date Fair Value		Number of Units		Weighted-Average Grant-Date Fair Value		Number of Units		Weighted-Average Grant-Date Fair Value
Unvested at beginning of year		93	$	43.78		45	$	46.21		—	$	—
Granted		6		55.17		48		41.48		45		46.21
Unvested at end of year		99	$	44.38		93	$	43.78		45	$	46.21

The following table summarizes activity for TSR LTIP II units for the years ended December 31, 2019 and 2018 (units in thousands):

	2019				2018
	Number of Units		Weighted-Average Grant-Date Fair Value		Number of Units		Weighted-Average Grant-Date Fair Value
Unvested at beginning of year		243	$	8.29		—	$	—
Granted		356		12.03		243		8.29
Unvested at end of year		599	$	10.51		243	$	8.29

We estimated the fair value of TSR-based awards granted in 2019, 2018, and 2017 using a Monte Carlo model with the assumptions set forth in the table below.

The risk-free interest rate reflects the annualized yield of a zero coupon United States Treasury security with a term equal to the expected term of the awards. The expected dividend yield reflects expectations regarding cash dividend amounts per share paid on Aimco’s Common Stock during the expected term of the awards. Expected volatility reflects an average of the historical volatility of Aimco’s Common Stock during the historical period commensurate with the expected term of the award that ended on the date of grant, and the implied volatility is calculated from observed call option contracts closest to the expected term. The derived vesting period of TSR Restricted Stock and TSR LTIP I units was determined based on the graded vesting terms. The expected term of the TSR-options and TSR LTIP II units was based on historical exercises and post-vesting terminations. The valuation assumptions for the 2019, 2018, and 2017 grants were as follows:

	2019			2018			2017
Grant date market value of a common share	$	49.24		$	40.95		$	44.07
Risk-free interest rate		2.59% - 2.66	%		2.32% - 2.68	%		1.57% - 2.22	%
Dividend yield		3.09	%		3.52	%		3.27	%
Expected volatility		19.08% - 19.24	%		17.64% - 18.02	%		21.33% - 23.00	%
Derived vesting period of TSR Restricted Stock and TSR LTIP I units	3.4 years			3.4 years			3.4 years
Weighted average expected term of TSR Stock Options and LTIP II units	5.8 years			5.6 years			5.8 years

The grant date fair value for the Time-Based Restricted Stock awards reflects the closing price of a share of Aimco Common Stock on the grant date.