Common Stock (Employee Stock Purchase Plan) (Details) - USD ($) $ in Thousands		12 Months Ended
	May 18, 2012	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 7,100	$ 10,600	$ 11,400
Total fair value of the shares purchased under the plan		400	800	1,000
Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Registered shares of common stock (in shares)	500,000
Percent of common stock fair market value	85.00%
Offering period	3 months
Maximum employee compensation payroll deductions may not exceed	10.00%
Maximum shares employees may purchase in any one offering period (in shares)	1,000
Maximum purchase rights granted to employee during calendar year	$ 25
Share-based compensation expense		$ 100	$ 100	$ 100