Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 288,039	$ 752,842	$ 369,526	$ 214,726	$ 261,086	$ 693,257	$ 229,905	$ 99,140	$ 1,625,133	$ 1,283,388	$ 665,783
Weighted-average common shares outstanding (in shares)									214,617	215,393	219,537
Dilutive effect of stock-based awards (in shares)									1,077	923	1,152
Diluted weighted-average shares outstanding									215,694	216,316	220,689
Basic earnings per share:
Basic earnings per share (in dollars per share)	$ 1.35	$ 3.51	$ 1.72	$ 1.00	$ 1.22	$ 3.23	$ 1.07	$ 0.46	$ 7.57	$ 5.96	$ 3.03
Diluted Earnings per Share:
Diluted earnings per share (in dollars per share)	$ 1.34	$ 3.49	$ 1.71	$ 0.99	$ 1.21	$ 3.21	$ 1.06	$ 0.46	$ 7.53	$ 5.93	$ 3.02