Unaudited Interim Financial Data - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 309,892	$ 279,408	$ 258,369	$ 235,157	$ 329,585	$ 266,212	$ 278,142	$ 362,976	$ 1,082,826	$ 1,236,915	$ 2,302,065
Operating loss	(21,745)	(21,843)	(845,606)	(56,880)	(110,294)	(56,604)	(44,285)	(45,463)	(946,074)	(165,720)	(760,197)
Net loss from continuing operations	$ (33,062)	$ (81,591)	$ (628,063)	$ (142,334)	(24,675)	(96,792)	(91,864)	(301,694)	(885,050)	(516,511)	(929,580)
Net loss from discontinued operations, net of tax					$ 0	$ 0	$ (1,486)	$ 0	$ 0	$ (1,486)	$ 0
Basic:
Loss from continuing operations (usd per share)	$ (0.13)	$ (0.33)	$ (2.55)	$ (0.58)	$ (0.10)	$ (0.40)	$ (0.37)	$ (1.24)	$ (3.59)	$ (2.10)	$ (3.82)
Loss from discontinued operations (usd per share)					0.00	0.00	(0.01)	0.00	0.00	(0.01)	0.00
Diluted:
Loss from continuing operations (usd per share)	$ (0.13)	$ (0.33)	$ (2.55)	$ (0.58)	(0.10)	(0.40)	(0.37)	(1.24)	(3.59)	(2.10)	(3.82)
Loss from discontinued operations (usd per share)					$ 0.00	$ 0.00	$ (0.01)	$ 0.00	$ 0.00	$ (0.01)	$ 0.00