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, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 1,042,852		$ 2,235,949		$ 1,987,013		$ 1,575,471		$ 1,662,267		$ 1,506,148		$ 1,481,493		$ 1,411,751		$ 1,428,625		$ 1,431,282		$ 1,375,937		$ 1,376,182		$ 6,841,285		$ 6,061,659		$ 5,612,026
Net income (loss)	(753,374)	[1]	409,028	[1]	279,587	[1]	(65,594)	[1]	439,326	[1]	(261,035)	[1]	151,427	[1]	71,040	[1]	132,703	[1]	83,421	[1]	80,673	[1]	163,646	[1]	(130,353)		400,758		460,443
Net income (loss) to shareholders	(751,543)	[1]	409,438	[1]	278,231	[1]	(64,306)	[1]	434,881	[1]	(259,141)	[1]	149,660	[1]	69,869	[1]	132,726	[1]	83,796	[1]	78,797	[1]	160,370	[1]	(128,180)	[2]	395,269	[2]	455,689	[2]
Comprehensive income (loss) to shareholders	$ (680,373)		$ 315,106		$ 164,336		$ (174,839)		$ 629,247		$ (19,869)		$ 342,357		$ 223,239		$ (29,050)		$ 89,161		$ 209,942		$ 396,994		$ (375,770)		$ 1,174,974		$ 667,047
Basic net income (loss) per share (dollars per share)	$ (53.88)		$ 28.56		$ 20.01		$ (4.25)		$ 30.48		$ (18.82)		$ 10.34		$ 3.91		$ 9.14		$ 5.62		$ 5.44		$ 11.21		$ (9.55)		$ 25.89		$ 31.41
Diluted net income (loss) per share (dollars per share)	$ (53.88)		$ 28.50		$ 19.97		$ (4.25)		$ 30.39		$ (18.82)		$ 10.31		$ 3.90		$ 9.11		$ 5.60		$ 5.41		$ 11.15		$ (9.55)	[3]	$ 25.81	[3]	$ 31.27	[3]

[1] Effective January 1, 2018, the Company adopted ASU No. 2016-01. As a result, the change in fair value of equity securities is no longer recognized in other comprehensive income; rather, all changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[2] Effective January 1, 2018, the Company adopted ASU No. 2016-01 and equity securities are no longer classified as available-for-sale with unrealized gains and losses recognized in other comprehensive income, rather, changes in the fair value of equity securities are now recognized in net income. Prior periods have not been restated to conform to the current presentation. See note 1.

[3] The impact of restricted stock units and restricted stock of 25 thousand shares was excluded from the computation of diluted earnings per share for the year ended December 31, 2018 because the effect would have been anti-dilutive.