Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 693,257	$ 228,787	$ 1,022,302	$ 458,984
Weighted-average common shares outstanding (shares)	214,819,000	219,963,000	215,663,000	219,835,000
Dilutive effect of stock options, performance share awards and restricted stock awards (shares)	848,000	1,174,000	912,000	1,144,000
Diluted weighted-average shares outstanding (shares)	215,667,000	221,137,000	216,575,000	220,979,000
Basic earnings per share (in dollars per share)	$ 3.23	$ 1.04	$ 4.74	$ 2.09
Diluted earnings per share (in dollars per share)	$ 3.21	$ 1.03	$ 4.72	$ 2.08
Antidilutive securities (shares)	0	0	0	0