Debt (Details) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)	Jan. 22, 2019 USD ($)
Debt instruments [Line Items]
Capital lease obligations		$ 0	[1]	$ 1,864
Long-term debt, principal amount		946,000		911,864
Current portion of long-term debt		0		791
Long-term debt		946,000		911,073
Long-term debt due within one year		0		791
Total debt		946,000		911,864
Revolving credit facility, current commitment		1,150,000		950,000
Line of credit facility increase in current capacity					$ 200,000
Revolving credit facility, maximum commitment		1,425,000
Revolving credit facility [Member]
Debt instruments [Line Items]
Debt, principal amount		946,000		$ 910,000
Revolving credit facility, current commitment		$ 1,150,000
Ratio of total debt less unrestricted cash to EBITDA		2.75
Weighted-average interest rate at period end		3.78%		3.79%
Maximum leverage ratio		3.5
Minimum ratio of EBIT to interest expense		3.0
Daily average amount outstanding		$ 936,583		$ 731,110
Weighted-average interest rate		3.76%		3.24%
Outstanding letters of credit	[2]	$ (5,868)
Net available for borrowing as of March 31, 2019		$ 198,132
Revolving credit facility [Member] | Minimum [Member]
Debt instruments [Line Items]
Revolving credit facility, commitment fee		0.175%
Revolving credit facility [Member] | Maximum [Member]
Debt instruments [Line Items]
Revolving credit facility, commitment fee		0.35%
Capital lease obligations [Member]
Debt instruments [Line Items]
Current portion of long-term debt		$ 0	[1]	$ 791
Term loan facility [Member]
Debt instruments [Line Items]
Daily average amount outstanding	[3]	$ 0		$ 63,638
Weighted-average interest rate	[3]	0.00%		2.97%

[1] Upon adoption of ASU No. 2016-02, Leasing, on January 1, 2019 (Note 2), we reclassified our capital lease obligations, now known as finance lease obligations, to accrued liabilities and other non-current liabilities on the consolidated balance sheet.

[2] We use standby letters of credit to collateralize certain obligations related primarily to our self-insured workers’ compensation claims, as well as claims for environmental matters, as required by certain states. These letters of credit reduce the amount available for borrowing under our revolving credit facility.

[3] During 2018, we had borrowings outstanding under a variable rate term loan facility. These amounts were repaid in March 2018.