Employee Benefits - Net Pension Cost and Amount Recognized in OCI for All Funded and Unfunded Plans (Details) - Pension Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Interest cost on PBO	$ 41	$ 48	$ 44
Expected return on plan assets	(53)	(68)	(58)
Amortization of losses	17	15	17
Settlement loss	17	0	18
Net pension cost (benefit)	22	(5)	21
Other changes in plan assets and benefit obligations recognized in OCI:
Net (gain) loss	20	(10)	(9)
Amortization of gains	(33)	(15)	(35)
Total recognized in comprehensive income	(13)	(25)	(44)
Total recognized in net pension cost and comprehensive income	$ 9	$ (30)	$ (23)