Lease Obligations - Financial Statement Line Items (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Lease Obligations
Operating lease assets	$ 769,487	$ 807,537
Finance lease assets	$ 29,756
Financial Statement Line Items	us-gaap:PropertyPlantAndEquipmentNet	us-gaap:PropertyPlantAndEquipmentNet
Total lease assets, net	$ 799,243	$ 807,537
Operating lease liabilities - current	62,405	$ 58,613
Finance lease liabilities - current	$ 2,240
Financial Statement Line Items	us-gaap:LinesOfCreditCurrent	us-gaap:LinesOfCreditCurrent
Operating lease liabilities - non-current	$ 804,555	$ 843,312
Finance lease liabilities - non-current	$ 27,742
Financial Statement Line Items	us-gaap:LongTermLineOfCredit	us-gaap:LongTermLineOfCredit
Total lease liabilities	$ 896,942	$ 901,925