Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 229,905	$ 184,967	$ 329,045	$ 230,197
Weighted-average common shares outstanding (shares)	215,265,000	219,913,000	216,089,000	219,770,000
Dilutive effect of stock options, performance share awards and restricted stock awards (shares)	866,000	989,000	951,000	1,116,000
Diluted weighted-average shares outstanding (shares)	216,131,000	220,902,000	217,040,000	220,886,000
Basic earnings per share (in dollars per share)	$ 1.07	$ 0.84	$ 1.52	$ 1.05
Diluted earnings per share (in dollars per share)	$ 1.06	$ 0.84	$ 1.52	$ 1.04
Antidilutive securities (shares)			0	0