Long-Term Debt and Capital Leases and Letters of Credit - Credit Facility and Capital Lease (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Total capital leases	$ 4,300,000	$ 6,700,000
Total capital leases, current	1,100,000	$ 1,300,000
Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit, outstanding	38,400,000
Credit facility | $750.0 million credit facility due December 2020, weighted average rate of 2.72% at December 31, 2017
Line of Credit Facility [Line Items]
Maximum borrowing capacity	750,000,000
Credit facility | Letter of Credit
Line of Credit Facility [Line Items]
Available borrowings capacity under letters of credit	$ 714,800,000