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Stock Based Compensation
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock Based Compensation Stock Based Compensation
We are authorized to issue up to 9.7 million shares of the General Partner's common stock under our stock-based employee and non-employee compensation plans. Executive officers may elect to receive Long-Term Incentive Plan Units ("LTIP Units"), which represent an interest in the Partnership, in lieu of stock based compensation awards denominated in the General Partner's common stock.
Restricted Stock Units ("RSUs")
Under our 2015 Long-Term Incentive Plan, which was approved by the General Partner's shareholders in April 2015, and our 2015 Non-Employee Directors Compensation Plan (collectively, the "Compensation Plans"), RSUs may be granted to non-employee directors, executive officers and selected employees. An RSU is economically equivalent to a share of the General Partner's common stock, and RSUs are valued based on the market price of the General Partner's common stock on the date of the award. Amounts disclosed below include both RSUs and any elected LTIP Units, which have the same vesting schedule as RSUs.
RSUs granted to employees from 2015 to 2021 vest ratably in most cases over a three-year period and are payable in shares of our common stock with a new share of such common stock issued upon each RSU's vesting. RSUs granted to existing non-employee directors vest 100% over one year and have contractual lives of one year.
To the extent that a recipient of an RSU grant is not determined to be retirement eligible, as defined by the Compensation Plans, we recognize expense on a straight-line basis over the vesting period. Expense is recognized immediately at the date of grant to the extent a recipient is retirement eligible and expense is accelerated to the extent that a participant will become retirement eligible prior to the end of the contractual life of granted RSUs.
The following table summarizes transactions for our unvested RSUs, excluding dividend equivalents, for 2021: 
Restricted Stock UnitsNumber of
RSUs		Weighted
Average
Grant-Date
Fair Value
December 31, 2020678,803 $32.98
Granted in 2021322,227 $42.15
Vested in 2021(368,428)$31.66
Forfeited in 2021(41,760)$37.47
December 31, 2021590,842 $38.49
Compensation cost recognized for RSUs totaled $12.5 million, $12.1 million and $11.0 million for the years ended December 31, 2021, 2020 and 2019, respectively.
As of December 31, 2021, there was $6.3 million of total unrecognized compensation expense related to nonvested RSUs granted under the Plan, which is expected to be recognized over a weighted average period of 1.7 years.
The total intrinsic value (which is equal to the value of a share of the General Partner's common stock on the date of vesting) of RSUs vested during the years ended December 31, 2021, 2020 and 2019 was $11.7 million, $15.4 million and $17.7 million, respectively.
The weighted average grant-date fair value of RSUs granted during 2020 and 2019 was $37.28 and $29.98, respectively.
The weighted average grant-date fair value of nonvested RSUs as of December 31, 2019 was $27.73.
Performance-Based Awards
A portion of the annual stock-based compensation awards granted to our executive officers annually include performance conditions, measured over a three-year performance period, based on pre-established goals for growth in a defined adjusted funds from operations (“AFFO”) metric. These performance-based awards disclosed below include awards denominated in both common shares of the General Partner or LTIP Units. The total number of instruments issued at the end of each performance period may be earned in a range from 0% to 200% of the target value of the award depending on our AFFO performance relative to the pre-established goals.
To the extent that a recipient of these performance-based awards is not determined to be retirement eligible, as defined by the Compensation Plans, we recognize expense on a straight-line basis over the performance period based on the most likely payout percentage at each reporting period for each grant to the extent that a payout is determined to be probable. Expense is recognized immediately at the date of grant, based on the most likely payout percentage to the extent that a payout is determined to be probable, when a recipient is retirement eligible, and expense is accelerated to the extent that a participant will become retirement eligible prior to the end of the performance period of an award.
Details on the unvested amounts of these annual grants by performance period are as follows:
Performance-Based AwardsUnvested Awards OutstandingUnvested Weighted Average Grant Date Fair Value
Unvested awards at December 31, 2020207,712 $33.58
Above target performance adjustment105,416 $29.98
Vested in 2021(210,832)$29.98
Granted in 202197,527 $42.07
Unvested awards at December 31, 2021199,823 $39.62

A summary of vested performance-based awards that are denominated in LTIP units is as follows:

Vested LTIP Awards Outstanding
Vested Awards at December 31, 2020322,569
Vested in 2021148,518
Completed holding period in 2021(142,324)
Vested Awards at December 31, 2021328,763
Compensation cost recognized for these performance-based awards totaled $8.5 million, $7.8 million and $6.2 million for the years ended December 31, 2021, 2020 and 2019, respectively.
As of December 31, 2021, there was $760,000 of total unrecognized compensation expense related to nonvested performance-based awards, which is expected to be recognized over a weighted average period of 1.5 years.
The weighted average grant-date fair value, per instrument, for these performance-based awards granted during 2020 and 2019 was $37.29 and $29.98.
The weighted average grant-date fair value of these nonvested performance-based awards as of December 31, 2019 was $27.50.