12. Common Stock

Common stock par value $0.01 per share, was authorized in the amount of 150,000 thousand shares, of which 42,564 thousand shares were issued and 34,190 thousand shares were outstanding as of December 31, 2017.

Changes in common stock for each period are as follows (in thousands):


	Year Ended December 31,
	2017					2016
	Shares			Amount		Shares		Amount
Common stock at the beginning of the period		35,048,338		$	416		34,568,942	$	411
Exercise of stock options		539,183			6		296,103		3
Settlement of restricted stock units		397,522			4		183,293		2
Acquisitions of treasury stock		(1,795,444	)		—		—		—

Total common stock outstanding at the end of the period		34,189,599		$	426		35,048,338	$	416