Retirement Plans (Schedule of Funded Status and Components of the Amounts Recognized in the Consolidated Balance Sheets and in Accumulated Other Comprehensive Gain (Loss), Before the Effect of Income Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of Net Periodic Pension Cost
Service cost	$ (42)	$ (33)	$ (36)
Interest cost	(10)	(11)	(10)
Expected return on plan assets	11	10	11
Amortization of prior service cost and transition amount	1	1	1
Recognized actuarial loss	(9)	(9)	(13)
Net periodic benefit cost	(49)	(42)	(47)
Net actuarial gain (loss)	(25)	(26)	26
Recognized net actuarial loss	9	9	13
Prior service cost and transition amount	(1)	(1)	(1)
Total recognized in OCI	(17)	(18)	38
Total recognized in net periodic benefit cost and OCI	$ (66)	$ (60)	$ (9)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate	1.80%	2.10%	2.00%
Expected return on plan assets	3.30%	3.60%	4.00%
Expected return on plan assets	2.80%	2.30%	2.90%
Weighted-average discount rate used to determine projected benefit obligations	1.80%	1.80%	2.10%
Defined Benefit Plan, Information about Plan Assets [Abstract]
Fair value of plan assets	$ 370	$ 308
Benefit obligations	(708)	(588)	$ (529)
Funded status	(338)	(280)
Current liabilities—accrued compensation	(2)	(1)
Noncurrent liabilities—other liabilities	(336)	(279)
Unrecognized net actuarial loss	(177)	(160)
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
Defined Benefit Plan, Information about Plan Assets [Abstract]
Unrecognized prior service credit	12	11
Unrecognized net actuarial loss	$ (189)	$ (171)