General - Long Term Debt and Capital Lease Obligation (Details) - USD ($) $ in Thousands		6 Months Ended
	Sep. 22, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Long-term Debt and Capital Lease Obligations
Proceeds from issuance of debt			$ 200,000
Debt disclosures
Capital lease obligations		$ 24,900		$ 22,900
Deferred loan issuance cost		6,000		6,600
4.400% Senior Notes due 2024
Long-term Debt and Capital Lease Obligations
Aggregate principal amount of Senior Notes	$ 400,000	399,300		399,300
Stated interest rate (as a percent)	4.40%
Redemption price as a percentage of the principal amount being redeemed	100.00%
2017 Credit Agreement
Debt disclosures
Letters of credit outstanding		17,600
Revolving borrowings outstanding		70,000		92,500
Available borrowing capacity		$ 312,400
2017 Credit Agreement | Revolving Loan borrowings
Long-term Debt and Capital Lease Obligations
Maximum borrowing capacity	$ 400,000
Commitment commission (as a percent)		0.20%
2017 Credit Agreement | Term Loan
Long-term Debt and Capital Lease Obligations
Maximum borrowing capacity	$ 350,000
Weighted average interest rate (as a percent)		3.392%
Long-term Debt, Fiscal Year Maturity
2018		$ 8,800
2019		17,500
2020		17,500
2021		17,500
2022		275,600
2017 Credit Agreement | Prime rate
Long-term Debt and Capital Lease Obligations
Basis spread on variable rate (as a percent)	0.50%
2017 Credit Agreement | Overnight Federal Funds rate
Long-term Debt and Capital Lease Obligations
Basis spread on variable rate (as a percent)	0.50%
2017 Credit Agreement | Eurodollar rate for one month
Long-term Debt and Capital Lease Obligations
Basis spread on variable rate (as a percent)	1.00%
2017 Credit Agreement | Eurodollar rate for selected interest period
Long-term Debt and Capital Lease Obligations
Basis spread on variable rate (as a percent)	1.50%
Standby Letters of Credit with The Bank Of Tokyo-Mitsubishi UFJ, Ltd.
Debt disclosures
Letters of credit outstanding		$ 48,500		$ 26,500