Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Component of periodic benefit cost
Defined Benefit Plan, Plan Assets, Contributions by Employer			$ 101
Pension Plan [Member]
Component of periodic benefit cost
Service cost	$ 1,000,000	$ 1,000,000	3,000,000	$ 3,000,000
Interest cost	(36,000,000)	(36,000,000)	(107,000,000)	(134,000,000)
Expected return on plan assets	(57,000,000)	(55,000,000)	(172,000,000)	(214,000,000)
Amortization of actuarial loss	(12,000,000)	(11,000,000)	(37,000,000)	(41,000,000)
Settlements	0		0	750,000,000
Net periodic cost (benefit)	(8,000,000)	(7,000,000)	(25,000,000)	714,000,000
Other Postretirement Benefits Plan [Member]
Component of periodic benefit cost
Service cost	0	0	0	6,000,000
Interest cost	(1,000,000)	(2,000,000)	(4,000,000)	6,000,000
Expected return on plan assets	(2,000,000)	(2,000,000)	(5,000,000)	(5,000,000)
Amortization of prior service credit	(2,000,000)	(2,000,000)	(5,000,000)	0
Amortization of actuarial loss	(2,000,000)	(1,000,000)	(5,000,000)	(4,000,000)
Settlements		0
Net periodic cost (benefit)	$ (1,000,000)	$ (1,000,000)	$ (1,000,000)	$ (1,000,000)