Stock-Based Compensation Plans - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - USD ($) $ / shares in Units, shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation Plans - Disclosures
Maximum value of company common stock that can be purchased by an employee per year	$ 25,000
Employee purchase price as a percent of fair market value (as a percent)	85.00%
Share purchases under employee stock purchase plan (in shares)	0.5	0.7	0.6
Weighted-average fair value of discount on employee stock purchase plan (in dollars per share)	$ 14.72	$ 14.00	$ 7.29
Intrinsic value from shares settled	$ 7,900,000	$ 10,200,000	$ 4,100,000
Cash received from shares issued	27,600,000	25,500,000	23,400,000
Tax benefits realized from the settlement of share-based payment arrangements	$ 2,000,000	$ 600,000	$ 600,000
Shares available to be issued under employee stock purchase plan (in shares)	2.9