Stockholders' Equity

Shares of our common stock outstanding and activity for the years ended December 31, 2017, 2016 and 2015 are presented below:



	Years Ended December 31,
	2017		2016		2015

Common shares outstanding at January 1	177,345,266		180,234,388		196,086,090
Issuance of restricted stock, excluding restricted stock with performance feature	55,626		870,378		447,534
Vesting of performance units	138,374		76,404		136,896
Vesting of restricted stock with performance feature	350,063		40,294		43,774
Forfeitures of restricted stock	(139,634	)	(16,795	)	(51,332	)
Purchase of treasury stock ⁽¹⁾	(342,073	)	(3,859,403	)	(16,428,574	)
Common shares outstanding at December 31	177,407,622		177,345,266		180,234,388


(1)	Includes 342,073, 147,922 and 151,967 shares, respectively, withheld under the terms of stock-based compensation agreements to provide funds for the payment of payroll and income taxes due at the vesting of share-based awards, as well as other stock repurchases under separate authority from our Board of Directors.

In May 2015, our Board of Directors approved a $1 billion share repurchase program, which replaced all existing share repurchase programs, authorizing us to repurchase common stock in the open market or through privately negotiated transactions. The timing and amount of stock repurchases will depend on market conditions and corporate, regulatory and other relevant considerations. This program may be discontinued at any time by the Board of Directors. As of December 31, 2017, we had remaining authorization to repurchase up to $178.8 million under this stock repurchase program. In addition, we are authorized by our Board of Directors to repurchase shares in an amount sufficient to offset shares issued under our compensation programs.

During the years ended December 31, 2017, 2016 and 2015, we withheld shares of our common stock from certain employees in the amounts of $15.9 million, $4.7 million and $6.2 million, respectively. These withholdings were made under the terms of restricted stock and performance share unit agreements upon vesting, at which time, we concurrently made cash payments to fund payroll and income taxes on behalf of officers and employees who elected to have shares withheld from vested amounts to pay such taxes.