Benefit Plans - Summary of the Components of Net Periodic Benefit Cost Recognized for Pension Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Periodic Pension Cost:
Service cost	$ 200	$ 186	$ 176
Interest cost	192	181	157
Estimated return on plan assets	(372)	(326)	(327)
Net amortization and other	75	80	67
Net periodic benefit cost	95	121	73
Pre-Tax Amounts Recognized in OCI:
Prior service credit (cost)	30	0	0
Net actuarial loss (gain)	137	138	230
Net amortization	(75)	(80)	(67)
Net amount recognized in OCI	92	58	163
Total net periodic pension costs (income) recognized in total comprehensive income, pre-tax	$ 187	$ 179	$ 236