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, 2016	Dec. 31, 2015
Fair value of plan assets in qualified defined benefit pension plan, Excluding assets valued at net asset value	$ 2,032	$ 1,820
Fair value of plan assets in qualified defined benefit pension plan	2,448	2,347
Cash equivalent securities, mutual funds | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan		43
Cash equivalent securities, mutual funds | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan	2
Equity securities | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan		69
Equity securities | Common stock
Fair value of plan assets in qualified defined benefit pension plan	850	480
Debt securities | U.S. Treasury and other U.S. government agency securities
Fair value of plan assets in qualified defined benefit pension plan	377	368
Debt securities | Corporate and municipal bonds and notes
Fair value of plan assets in qualified defined benefit pension plan	709	729
Debt securities | Collateralized mortgage obligations
Fair value of plan assets in qualified defined benefit pension plan	15	18
Debt securities | U.S. government agency mortgage-backed securities
Fair value of plan assets in qualified defined benefit pension plan	8	8
Debt securities | Collective investments funds
Fair value of plan assets in qualified defined benefit pension plan		527
Private placements
Fair value of plan assets in qualified defined benefit pension plan	71	105
Level 1
Fair value of plan assets in qualified defined benefit pension plan, Excluding assets valued at net asset value	1,227	958
Level 1 | Cash equivalent securities, mutual funds | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan		43
Level 1 | Cash equivalent securities, mutual funds | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan	0
Level 1 | Equity securities | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan		69
Level 1 | Equity securities | Common stock
Fair value of plan assets in qualified defined benefit pension plan	850	478
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	377	368
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 | Debt securities | U.S. government agency mortgage-backed securities
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	734	757
Level 2 | Cash equivalent securities, mutual funds | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan		0
Level 2 | Cash equivalent securities, mutual funds | Commercial paper
Fair value of plan assets in qualified defined benefit pension plan	2
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	2
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	0	0
Level 2 | Debt securities | Corporate and municipal bonds and notes
Fair value of plan assets in qualified defined benefit pension plan	709	729
Level 2 | Debt securities | Collateralized mortgage obligations
Fair value of plan assets in qualified defined benefit pension plan	15	18
Level 2 | Debt securities | U.S. government agency mortgage-backed securities
Fair value of plan assets in qualified defined benefit pension plan	8	8
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	71	105
Level 3 | Cash equivalent securities, mutual funds | Mutual funds
Fair value of plan assets in qualified defined benefit pension plan		0
Level 3 | Cash equivalent securities, mutual funds | 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 | Debt securities | U.S. government agency mortgage-backed securities
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	$ 71	$ 105