Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic EPS:
Net income	$ 325	$ 372	$ 556	$ 159	$ (583)	$ 661	$ 864	$ 340	$ 1,412	$ 1,282	$ 2,584
Weighted average common shares outstanding (in shares)	460,589	460,526	463,533	472,297	477,165	484,772	490,818	503,902	464,236	489,164	552,308
Basic EPS (in dollars per share)	$ 0.71	$ 0.81	$ 1.20	$ 0.34	$ (1.22)	$ 1.36	$ 1.76	$ 0.67	$ 3.04	$ 2.62	$ 4.68
Diluted EPS:
Net income for purposes of computing diluted EPS									$ 1,412	$ 1,282	$ 2,584
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)	460,589	460,526	463,533	472,297	477,165	484,772	490,818	503,902	464,236	489,164	552,308
Dilutive effect of stock awards (in shares)									1,424	2,528	3,791
Diluted weighted average common shares outstanding (in shares)	461,915	461,507	464,618	474,598	477,165	486,625	492,965	507,797	465,660	491,692	556,099
Diluted EPS (in dollars per share)	$ 0.70	$ 0.81	$ 1.20	$ 0.34	$ (1.22)	$ 1.36	$ 1.75	$ 0.67	$ 3.03	$ 2.61	$ 4.65
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in shares)									1,266	328	1,429