Stockholders' Equity - Common stock repurchased (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Shares repurchased related to employee stock-based compensation plans (in shares)	200,577	981,477	2,940,595	2,584,964
Average purchase price per share (in dollars per share)	$ 11.60	$ 12.39	$ 11.33	$ 12.12
Common shares issued (in shares)	384,955	1,491,057	5,944,946	5,229,774