Retirement Plans (Schedule of Fair Value of Pension Assets By Asset Class) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 112	$ 120	$ 112
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	100.00%	100.00%
Fair value of plan assets	$ 985	$ 1,059	$ 1,149
Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	319	500
Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	554	439
Pension Benefits [Member] | Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 112	$ 120
Corporate Bonds [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	30.00%	48.00%
Fair value of plan assets	$ 297	$ 507
Corporate Bonds [Member] | Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	205	371
Corporate Bonds [Member] | Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 92	$ 136
Other Bonds [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	4.00%	3.00%
Fair value of plan assets	$ 38	$ 35
Other Bonds [Member] | Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 38	$ 35
Short-Term Investments [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	3.00%	3.00%
Fair value of plan assets	$ 24	$ 32
Short-Term Investments [Member] | Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	6
Short-Term Investments [Member] | Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 16	$ 26
United States Treasuries [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	35.00%	17.00%
Fair value of plan assets	$ 343	$ 179
United States Treasuries [Member] | Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	68	88
United States Treasuries [Member] | Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 275	$ 91
Fixed Income Securities [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	69.00%	68.00%
Fair value of plan assets	$ 678	$ 721
Fixed Income Securities [Member] | Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	311	494
Fixed Income Securities [Member] | Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 367	$ 227
Equity Securities [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	17.00%	18.00%
Fair value of plan assets	$ 171	$ 186
Equity Securities [Member] | Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 171	$ 186
Hedge Fund And Alternative Investments [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	11.00%	11.00%
Fair value of plan assets	$ 112	$ 120
Hedge Fund And Alternative Investments [Member] | Pension Benefits [Member] | Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 112	$ 120
Other Securities [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	14.00%	14.00%
Fair value of plan assets	$ 136	$ 152
Other Securities [Member] | Pension Benefits [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	6
Other Securities [Member] | Pension Benefits [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	16	26
Other Securities [Member] | Pension Benefits [Member] | Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 112	$ 120