Commitments and Contingencies (401(k) Retirement Plan )(Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Compensation expense related to 401(k) retirement plan	$ 1.0	$ 1.0	$ 1.0
Up to 2 Percent
Defined Contribution Plan Disclosure [Line Items]
Company match	100.00%
Employee contribution	2.00%
From 4 to 8 Percent
Defined Contribution Plan Disclosure [Line Items]
Company match	50.00%
From 4 to 8 Percent | Minimum
Defined Contribution Plan Disclosure [Line Items]
Employee contribution	4.00%
From 4 to 8 Percent | Maximum
Defined Contribution Plan Disclosure [Line Items]
Employee contribution	8.00%