Employee Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of employer contribution to match participant contribution in defined contribution plan, net	100.00%
Cost of defined benefit plans	$ 471	$ 444	$ 432
Defined benefit plan cost	14	21	25
Defined benefit plan obligation	$ 118	111
Defined Contribution Benefit Plans [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of employer contribution to match participant contribution in defined contribution plan	3.00%
Defined Contribution Benefit Plans [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Percentage of employer contribution to match participant contribution in defined contribution plan	9.00%
Restoration Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of hours of service required to qualify for the plan	1,000 or more hours
Noncontributory and nonqualified plan, benefit expense	$ 40	20	20
Noncontributory and nonqualified plan, accrued benefits liabilities	201	175
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan cost	28	22	$ 33
Defined benefit plan obligation	$ 223	$ 222