Employee Benefit Plans Narrative (Details) - USD ($) $ in Millions		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Benefit Plans [Line Items]
Matching contributions recorded in Compensation and related expense		$ 5.0	$ 4.8	$ 3.9
Other Postretirement Benefits Plan [Member]
Employee Benefit Plans [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		4.00%
Number of hours of service for vesting, per year		1,000
Number of years of service for vesting		2 years
Annual vesting percentage		20.00%
Number of years for incremental percentage vesting until participant fully vested in company's matching contribution		4 years
Other Postretirement Benefits Plan [Member] | First Salary Deferral Contribution [Member]
Employee Benefit Plans [Line Items]
Employee contribution percentage		2.00%
Other Postretirement Benefits Plan [Member] | Second Salary Deferral Contribution [Member]
Employee Benefit Plans [Line Items]
Employee contribution percentage		4.00%
Other Postretirement Benefits Plan [Member] | Minimum [Member]
Employee Benefit Plans [Line Items]
Employee contribution percentage		1.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		50.00%
Other Postretirement Benefits Plan [Member] | Maximum [Member]
Employee Benefit Plans [Line Items]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match		100.00%
Employee Stock [Member]
Employee Benefit Plans [Line Items]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		10.00%
Maximum number of shares of Class B common stock for Employee Stock Purchase Plan		750,000
Defined Contribution Plan Number Of Shares Of Class B Common Stock Purchased Under Plan During Current Year		8,887
Defined Contribution Plan Number Of Shares Of Class B Common Stock Purchased Under Plan Since Inception		185,518
Subsequent Event [Member] | Other Postretirement Benefits Plan [Member]
Employee Benefit Plans [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	4.50%
Subsequent Event [Member] | Other Postretirement Benefits Plan [Member] | First Salary Deferral Contribution [Member]
Employee Benefit Plans [Line Items]
Employee contribution percentage	3.00%
Subsequent Event [Member] | Other Postretirement Benefits Plan [Member] | Second Salary Deferral Contribution [Member]
Employee Benefit Plans [Line Items]
Employee contribution percentage	3.00%
Subsequent Event [Member] | Other Postretirement Benefits Plan [Member] | Minimum [Member]
Employee Benefit Plans [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Subsequent Event [Member] | Other Postretirement Benefits Plan [Member] | Maximum [Member]
Employee Benefit Plans [Line Items]
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%