Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income for basic and diluted earnings per share	$ 206,799	$ 228,787	$ 184,967	$ 45,230	$ 109,768	$ 490,248	$ 137,673	$ 26,457	$ 665,783	$ 764,146	$ 473,692
Weighted-average common shares outstanding (in shares)									219,537	221,658	219,638
Dilutive effect of stock options, performance stock awards and restricted stock awards (in shares)									1,152	1,386	1,303
Diluted weighted-average shares outstanding									220,689	223,044	220,941
Basic earnings per share (in dollars per share)	$ 0.95	$ 1.04	$ 0.84	$ 0.21	$ 0.50	$ 2.20	$ 0.62	$ 0.12	$ 3.03	$ 3.45	$ 2.16
Diluted earnings per share (in dollars per share)	$ 0.94	$ 1.03	$ 0.84	$ 0.20	$ 0.49	$ 2.19	$ 0.62	$ 0.12	$ 3.02	$ 3.43	$ 2.14