Stock-Based Compensation Plans - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan shares in Millions	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Stock-Based Compensation Plans - Disclosures
Share purchases under employee stock purchase plan (in shares)	0.3
Weighted-average fair value of discount on employee stock purchase plan (in dollars per share) | $ / shares	$ 14.28
Shares available to be issued under employee stock purchase plan (in shares)	3.1