Unaudited Quarterly Financial Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Operating Revenues and Other	$ 2,965,498		$ 2,245,484		$ 1,103,374		$ 4,717,692		$ 4,320,246		$ 4,303,455		$ 4,697,630		$ 4,058,642		$ 11,032,048	$ 17,379,973	$ 17,275,399
Operating Income	487,662		(2,714)		(1,086,549)		57,585		863,751		827,959		1,130,771		876,530		(544,016)	3,699,011	4,469,346
Income (Loss) Before Income Taxes	427,760		(52,555)		(1,145,262)		31,003		831,208		797,457		1,089,366		827,236
Income Tax Provision (Benefit)	90,294		(10,088)		(235,878)		21,190		194,687		182,335		241,525		191,810		(134,482)	810,357	821,958
Net Income (Loss)	$ 337,466		$ (42,467)		$ (909,384)		$ 9,813		$ 636,521		$ 615,122		$ 847,841		$ 635,426		$ (604,572)	$ 2,734,910	$ 3,419,040
Net Income (Loss) Per Share
Basic (in dollars per share)	$ 0.58	[1]	$ (0.07)	[1]	$ (1.57)	[1]	$ 0.02	[1]	$ 1.10	[1]	$ 1.06	[1]	$ 1.47	[1]	$ 1.10	[1]	$ (1.04)	$ 4.73	$ 5.93
Diluted (in dollars per share)	$ 0.58	[1]	$ (0.07)	[1]	$ (1.57)	[1]	$ 0.02	[1]	$ 1.10	[1]	$ 1.06	[1]	$ 1.46	[1]	$ 1.10	[1]	$ (1.04)	$ 4.71	$ 5.89
Average Number of Common Shares [Abstract]
Basic (in shares)	579,624		579,055		578,719		578,462		578,219		577,839		577,460		577,207		578,949	577,670	576,578
Diluted (in shares)	580,885		579,055		578,719		580,283		580,849		581,271		580,247		580,222		578,949	580,777	580,441

[1] The sum of quarterly net income (loss) per share may not agree with total year net income (loss) per share as each quarterly computation is based on the weighted average of common shares outstanding.