Common Stock (Share-Based Compensation, Treasury Stock, and Stock Repurchase Plan) (Details) - USD ($)	12 Months Ended			43 Months Ended	74 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2018	Jun. 30, 2015	Nov. 30, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-cash share-based compensation expense	$ 7,100,000	$ 10,600,000	$ 11,400,000
Common stock shares purchased as payment of income tax withholding (in shares)	102,333	199,644	238,216
Stock surrendered for exercise of stock options (in shares)	0	478,287	3,225
Common stock shares purchased, cost		$ 5,203,000
Common stock shares purchased by the company (in shares)	0
Remaining authorized repurchase amount	$ 49,700,000			$ 49,700,000	$ 49,700,000
Share Repurchase Program, November 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchase shares authorized							$ 25,000,000
Common stock shares purchased, cost		$ 5,200,000			$ 25,000,000
Common stock shares purchased by the company (in shares)		905,000	0
Common stock shares purchased, average cost per share (in dollars per share)		$ 5.75
Share Repurchase Program, June 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock repurchase shares authorized						$ 50,000,000
Common stock shares purchased, cost				$ 300,000