Lease Obligations - Financial Statement Line Items (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Lease Obligations
Operating lease assets	$ 789,539	$ 807,537
Finance lease assets	$ 29,036
Financial Statement Line Items	us-gaap:PropertyPlantAndEquipmentNet
Total lease assets, net	$ 818,575	807,537
Operating lease liabilities - current	60,315	58,613
Finance lease liabilities - current	$ 1,991
Financial Statement Line Items	us-gaap:LinesOfCreditCurrent
Operating lease liabilities - non-current	$ 823,929	843,312
Finance lease liabilities - non-current	$ 26,201
Financial Statement Line Items	us-gaap:LongTermLineOfCredit
Total lease liabilities	$ 912,436	$ 901,925