Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 369,526	$ 229,905	$ 584,252	$ 329,045
Weighted-average common shares outstanding (shares)	215,085,000	215,265,000	214,978,000	216,089,000
Dilutive effect of stock options, performance share awards and restricted stock awards (shares)	977,000	866,000	966,000	951,000
Diluted weighted-average shares outstanding (shares)	216,062,000	216,131,000	215,944,000	217,040,000
Basic earnings per share (in dollars per share)	$ 1.72	$ 1.07	$ 2.72	$ 1.52
Diluted earnings per share (in dollars per share)	$ 1.71	$ 1.06	$ 2.71	$ 1.52
Antidilutive securities (shares)	0	0	0	0