Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased					3,000					3,000
Stock Repurchased During Period, Shares									800
Stock Repurchased During Period, Value									$ 32,200
Dilutive Impact of Common Stock Equivalents on Earnings per Share (Abstract]
Net income	$ 227,477	$ 457,795	$ 196,795	$ 241,312	$ 209,025	$ 426,463	$ 196,931	$ 152,066	1,123,379	$ 984,485	$ 1,021,306
Basic earnings per share [Abstract]
Earnings available to common shareholders									$ 1,123,379	$ 984,485	$ 1,021,306
Weighted average common shares outstanding - basic (in shares)									508,794	507,768	503,847
Earnings available to common shareholders - basic (in dollars per share)	$ 0.45	$ 0.90	$ 0.39	$ 0.47	$ 0.41	$ 0.84	$ 0.39	$ 0.30	$ 2.21	$ 1.94	$ 2.03
Effect of dilutive securities [Abstract]
401(k) equity awards (in shares)									671	400	270
Diluted earnings per share [Abstract]
Earnings available to common shareholders									$ 1,123,379	$ 984,485	$ 1,021,306
Weighted average common shares outstanding - diluted (in shares)									509,465	508,168	504,117
Earnings available to common shareholders - diluted (in dollars per share)	$ 0.45	$ 0.90	$ 0.39	$ 0.47	$ 0.41	$ 0.84	$ 0.39	$ 0.30	$ 2.21	$ 1.94	$ 2.03