Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 228,787	$ 490,248	$ 458,984	$ 654,378
Weighted-average common shares outstanding (shares)	219,963	222,523	219,835	222,007
Dilutive effect of stock options, performance share awards and restricted stock awards (shares)	1,174	1,336	1,144	1,344
Diluted weighted-average shares outstanding (shares)	221,137	223,859	220,979	223,351
Basic earnings per share (in dollars per share)	$ 1.04	$ 2.20	$ 2.09	$ 2.95
Diluted earnings per share (in dollars per share)	$ 1.03	$ 2.19	$ 2.08	$ 2.93