Quarterly Financial Data (Unaudited) (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares item shares	Sep. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Quarterly Financial Data (Unaudited)
Operating revenues	$ 797,158	$ 831,936	$ 809,759	$ 765,415	$ 758,017	$ 799,776	$ 801,338	$ 762,075	$ 3,204,268	$ 3,121,206	$ 3,095,563
Operating income (loss)	92,939	112,369	106,596	76,295	(404,464)	85,834	84,137	61,809	388,199	(172,684)	234,515
Net income (loss)	$ 289,928	$ 53,716	$ 50,477	$ 34,786	$ (270,244)	$ 41,322	$ 40,244	$ 27,092	$ 428,907	$ (161,586)	$ 117,817
Net income (loss) per common share:
Basic (in dollars per share) | $ / shares	$ 5.60	$ 1.04	$ 0.98	$ 0.67	$ (5.22)	$ 0.80	$ 0.78	$ 0.53	$ 8.28	$ (3.14)	$ 2.31
Diluted (in dollars per share) | $ / shares	$ 5.46	$ 1.01	$ 0.95	$ 0.65	$ (5.22)	$ 0.79	$ 0.77	$ 0.52	$ 8.08	$ (3.14)	$ 2.27
Weighted average common shares:
Basic (in shares) | shares	51,811	51,833	51,751	51,820	51,757	51,627	51,418	51,218	51,804	51,505	51,077
Diluted (in shares) | shares	53,140	53,080	52,977	53,202	51,757	52,471	52,194	52,014	53,100	51,505	51,825
Revaluation of deferred tax liablity									$ 246,800
Special Charge					$ 465,600					$ 465,649
Number of seats on aircraft | item					50