Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 184,967	$ 137,673	$ 230,197	$ 164,130
Weighted-average common shares outstanding (shares)	219,913	222,189	219,770	221,745
Dilutive effect of stock options, performance share awards and restricted stock awards (shares)	989	1,192	1,116	1,310
Diluted weighted-average shares outstanding (shares)	220,902	223,381	220,886	223,055
Basic earnings per share (in dollars per share)	$ 0.84	$ 0.62	$ 1.05	$ 0.74
Diluted earnings per share (in dollars per share)	$ 0.84	$ 0.62	$ 1.04	$ 0.74