Employee Stock Benefit Plans (401(k) Plan Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Defined Contribution Plans Disclosures [Line Items]
Annual limit on eligible earnings imposed by Internal Revenue Code	$ 270,000
Total matching contribution to all Plans	$ 3,600,000	$ 3,400,000	$ 3,400,000
Salary Plan
Schedule of Defined Contribution Plans Disclosures [Line Items]
Minimum annual contribution per participating employee, percent	1.00%
Maximum annual contribution per employee, percent	50.00%
Employer matching contribution, percent of match	100.00%
Employer matching contribution, percent	6.00%
Maximum annual employer matching contributions per employee	$ 16,200
Hourly Plan
Schedule of Defined Contribution Plans Disclosures [Line Items]
Minimum annual contribution per participating employee, percent	1.00%
Maximum annual contribution per employee, percent	25.00%
Employer matching contribution, percent of match	50.00%
Employer matching contribution, percent	6.00%
Maximum annual employer matching contributions per employee	$ 8,100