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
Dilutive Impact of Common Stock Equivalents on Earnings per Share (Abstract]
Net income	$ 209,025	$ 426,463	$ 196,931	$ 152,066	$ 196,339	$ 368,582	$ 195,164	$ 261,221	$ 984,485	$ 1,021,306	$ 948,234
Basic earnings per share [Abstract]
Earnings available to common shareholders									$ 984,485	$ 1,021,306	$ 948,234
Weighted average common shares outstanding - basic (in shares)									507,768	503,847	496,073
Earnings available to common shareholders - basic (in dollars per share)	$ 0.41	$ 0.84	$ 0.39	$ 0.30	$ 0.39	$ 0.73	$ 0.39	$ 0.52	$ 1.94	$ 2.03	$ 1.91
Effect of dilutive securities [Abstract]
401(k) equity awards (in shares)									400	270	459
Diluted earnings per share [Abstract]
Earnings available to common shareholders									$ 984,485	$ 1,021,306	$ 948,234
Weighted average common shares outstanding - diluted (in shares)									508,168	504,117	496,532
Earnings available to common shareholders - diluted (in dollars per share)	$ 0.41	$ 0.84	$ 0.39	$ 0.30	$ 0.39	$ 0.73	$ 0.39	$ 0.52	$ 1.94	$ 2.03	$ 1.91