Stockholders' Equity - Common stock repurchased (Details) - $ / shares	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Equity [Abstract]
Shares repurchased related to employee stock-based compensation plans (in shares)	981,477	263,218	2,584,964	1,391,927
Average purchase price per share (in dollars per share)	$ 12.39	$ 6.68	$ 12.12	$ 6.12
Common shares issued (in shares)	1,491,057	425,495	5,229,774	3,166,474