Stockholders' Equity (Details Textuals 1) - 401(k) Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Benefit Plans Disclosures [Line Items]
Employer contribution rate	100.00%
Employer's matching contributions	$ 7.1	$ 7.7	$ 10.1
Maximum
Defined Contribution Benefit Plans Disclosures [Line Items]
Employee contribution rate	6.00%