Annual report pursuant to Section 13 and 15(d)

Employee Benefit Plan (Details)

v3.8.0.1
Employee Benefit Plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Retirement Benefits [Abstract]      
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent 100.00%    
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay 6.00%    
Defined Contribution Plan, Contributions $ 7,000,000 $ 6,000,000 $ 5,000,000
Defined Contribution Plan, Employer Discretionary Contribution Amount $ 0