Stockholders' Equity - Common stock repurchased (Details) - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Equity [Abstract]
Shares repurchased related to employee stock-based compensation plans (in shares)	2,740,018	1,603,487
Average purchase price per share (in dollars per share)	$ 11.31	$ 11.96
Common shares issued (in shares)	5,559,991	3,738,717