Employment Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employment Benefits [Abstract]
Employee contribution limit per calendar year for employees under 50 years of age	$ 18,000
Employee contribution limit per calendar year for employees over 50 years of age	24,000
Matching contributions	$ 3,000,000	$ 1,700,000	$ 1,500,000