Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 261,086	$ 693,257	$ 229,905	$ 99,140	$ 206,799	$ 228,787	$ 184,967	$ 45,230	$ 1,283,388	$ 665,783	$ 764,146
Weighted-average common shares outstanding (in shares)									215,393	219,537	221,658
Dilutive effect of stock options, performance stock awards and restricted stock awards (in shares)									923	1,152	1,386
Diluted weighted-average shares outstanding									216,316	220,689	223,044
Basic earnings per share (in dollars per share)	$ 1.22	$ 1.04	$ 1.07	$ 0.46	$ 0.94	$ 1.04	$ 0.84	$ 0.21	$ 5.96	$ 3.03	$ 3.45
Diluted earnings per share (in dollars per share)	$ 1.21	$ 1.03	$ 1.06	$ 0.46	$ 0.95	$ 1.03	$ 0.84	$ 0.20	$ 5.93	$ 3.02	$ 3.43