Stockholders' Equity - Common stock repurchased (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Shares repurchased related to employee stock-based compensation plans (in shares)	81,817	371,165	2,666,781	1,763,092
Average purchase price per share (in dollars per share)	$ 10.93	$ 7.22	$ 12.09	$ 6.35
Common shares issued (in shares)	423,112	561,100	5,652,886	3,727,574