Stockholders' Equity

Shares of our common stock outstanding and activity for the years ended December 31, 2018, 2017 and 2016 are presented below:



	Years Ended December 31,
	2018		2017		2016

Common shares outstanding at January 1	177,407,622		177,345,266		180,234,388
Issuance of restricted stock, excluding restricted stock with performance feature	—		55,626		870,378
Vesting of performance units	115,596		138,374		76,404
Vesting of restricted stock with performance feature	543,396		350,063		40,294
Forfeitures of restricted stock	(58,497	)	(139,634	)	(16,795	)
Purchase of treasury stock ⁽¹⁾	(5,886,626	)	(342,073	)	(3,859,403	)
Common shares outstanding at December 31	172,121,491		177,407,622		177,345,266


(1)	Includes 369,255, 342,073 and 147,922 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 September 2018, 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, 2018, we had remaining authorization to repurchase up to $787.6 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, 2018, 2017 and 2016, we withheld shares of our common stock from certain employees in the amounts of $19.6 million, $15.9 million and $4.7 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.