Stock-Based Compensation

6 Months Ended

Jun. 30, 2018

A summary of restricted stock transactions during the six months ended June 30, 2018 is as follows:



	Number of shares
Non-vested stock awards outstanding at January 1, 2018	1,053,842
Awarded:
Time-vesting shares	1,493,093
Performance-based shares	142,154
Released:
Time-vesting shares	(353,555	)
Performance-based shares	—
Forfeitures:
Time-vesting shares	(7,300	)
Performance-based shares	(159,738	)
Non-vested stock awards outstanding at June 30, 2018	2,168,496

The performance-based share awards are subject to either a market condition or a performance condition. For awards subject to a market condition, the performance metric is the Company's total shareholder return, which includes share price appreciation and dividends paid during the three-year term of the award, measured against a peer group of companies. These awards cliff vest at the end of three years. The number of performance-based shares that will ultimately vest is based on the level of market performance achieved, ranging between 0% and 200% of the shares originally awarded, and are settled in stock.

For awards subject to a performance condition, the performance metric is the Company's average return on invested capital over the three-year term. These awards cliff vest at the end of three years. The number of performance-based shares that will ultimately vest is based on the level of performance achieved, ranging between 0% and 200% of the shares originally awarded, and are settled in stock.

The time-vesting restricted and deferred stock awards vest ratably over three to five years.

The weighted average grant-date fair values of awards issued during the six months ended June 30, 2018 were $44.51 for the time-vesting stock awards and $58.77 for the performance-based stock awards.

The fair value of the performance-based shares subject to a market condition awarded in 2018 was calculated using a Monte Carlo valuation model, including a weighted average stock price volatility of 19.9%, an expected term of three years, and a weighted average risk-free interest rate of 2.01%.

As of June 30, 2018, unrecognized compensation cost related to outstanding non-vested stock awards was $75. The weighted average period over which the expense is expected to be recognized is 3.9 years. The aggregate market value of the shares released on the vesting dates was $16 for the six months ended June 30, 2018.