Retirement Plans (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Contributions to defined contribution plans	$ 60	$ 64	$ 79
Projected benefit obligation	239	234
Accumulated benefit obligation in excess of plan assets	225	211
Expected benefit plan payments for each of the next five years	76
Benefit plan payments expected to be funded from cash and cash equivalents	3
Expected total benefit plan payments for five years after the next five years	406
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,035	985	$ 1,059
Net actuarial loss and prior service cost to be amortized from AOCI into net periodic benefit cost the next fiscal year	$ 14
Pension Benefits [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	70.00%
Pension Benefits [Member] | Fixed Income Securities [Member] | Level 1 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 342	311
Pension Benefits [Member] | Fixed Income Securities [Member] | Level 2 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 401	367
Pension Benefits [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	20.00%
Fair value of plan assets	$ 157	171
Pension Benefits [Member] | Other Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocations	10.00%
Fair value of plan assets	$ 135	$ 136
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss and prior service cost to be amortized from AOCI into net periodic benefit cost the next fiscal year	$ 1
Defined benefit plan health care cost trend rate assumed for next fiscal year	7.30%
Defined benefit plan ultimate health care cost trend rate	5.00%