Operating Segments and Related Disclosures - Geographic Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue	$ 667,007	$ 687,231	$ 701,682	$ 591,746	$ 578,351	$ 621,329	$ 584,133	$ 538,143	$ 2,647,666	$ 2,321,956	$ 2,141,463
DOT—Domestic
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue									2,435,900	2,136,200	1,940,200
DOT—Latin America
Revenues from External Customers and Long-Lived Assets [Line Items]
Operating revenue									$ 211,800	$ 185,800	$ 201,300