Employee Benefit Plans (Fair Values of the Corporation's Qualified Defined Benefit Pension Plan Investments Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value of plan assets in qualified defined benefit pension plan, Excluding assets valued at net asset value	$ 2,480	$ 2,070
Fair value of plan assets in qualified defined benefit pension plan	2,949	2,462
Cash equivalent securities | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan		3
Equity securities | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan	2
Equity securities | Common stock
Fair value of plan assets in qualified defined benefit pension plan	1,086	803
Debt securities | U.S. Treasury and other U.S. government agency securities
Fair value of plan assets in qualified defined benefit pension plan	574	496
Debt securities | Corporate and municipal bonds and notes
Fair value of plan assets in qualified defined benefit pension plan	734	679
Debt securities | Collateralized mortgage obligations
Fair value of plan assets in qualified defined benefit pension plan	27	29
Debt securities | Collective investments funds
Fair value of plan assets in qualified defined benefit pension plan	469	392
Private placements
Fair value of plan assets in qualified defined benefit pension plan	57	60
Level 1
Fair value of plan assets in qualified defined benefit pension plan, Excluding assets valued at net asset value	1,639	1,288
Level 1 | Cash equivalent securities | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan		3
Level 1 | Equity securities | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan	2
Level 1 | Equity securities | Common stock
Fair value of plan assets in qualified defined benefit pension plan	1,086	803
Level 1 | Debt securities | U.S. Treasury and other U.S. government agency securities
Fair value of plan assets in qualified defined benefit pension plan	551	482
Level 1 | Debt securities | Corporate and municipal bonds and notes
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 1 | Debt securities | Collateralized mortgage obligations
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 1 | Private placements
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 2
Fair value of plan assets in qualified defined benefit pension plan, Excluding assets valued at net asset value	784	722
Level 2 | Cash equivalent securities | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan		0
Level 2 | Equity securities | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan	0
Level 2 | Equity securities | Common stock
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 2 | Debt securities | U.S. Treasury and other U.S. government agency securities
Fair value of plan assets in qualified defined benefit pension plan	23	14
Level 2 | Debt securities | Corporate and municipal bonds and notes
Fair value of plan assets in qualified defined benefit pension plan	734	679
Level 2 | Debt securities | Collateralized mortgage obligations
Fair value of plan assets in qualified defined benefit pension plan	27	29
Level 2 | Private placements
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 3
Fair value of plan assets in qualified defined benefit pension plan, Excluding assets valued at net asset value	57	60
Level 3 | Cash equivalent securities | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan		0
Level 3 | Equity securities | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan	0
Level 3 | Equity securities | Common stock
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 3 | Debt securities | U.S. Treasury and other U.S. government agency securities
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 3 | Debt securities | Corporate and municipal bonds and notes
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 3 | Debt securities | Collateralized mortgage obligations
Fair value of plan assets in qualified defined benefit pension plan	0	0
Level 3 | Private placements
Fair value of plan assets in qualified defined benefit pension plan	$ 57	$ 60