Stockholders' Equity - Common stock repurchased (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Common stock repurchased (in shares)	5,435,476	0
Average purchase price per share, including commission costs (in usd per share)	$ 11.04	$ 0.00
Shares repurchased related to employee stock-based compensation plans (in shares)	1,289,391	2,740,018
Average purchase price per share (in usd per share)	$ 10.95	$ 11.31
Issuance of common shares (in shares)	3,470,664	5,559,991
Average purchase price per share under repurchase program	$ 140