Stockholders' Equity - Common Stock Repurchased (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Shares repurchased related to employee stock-based compensation plans (in shares)	3,358,417	3,354,730	3,008,913
Average purchase price per share (in dollars per share)	$ 11.96	$ 7.83	$ 9.65
Common shares issued (in shares)	6,831,108	5,955,045	5,873,309