Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Component of periodic benefit cost
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 70
Pension Plan [Member]
Component of periodic benefit cost
Service cost	1,000,000	$ 1,000,000	$ 3,000,000	$ 3,000,000
Interest cost	(40,000,000)	(36,000,000)	(119,000,000)	(107,000,000)
Expected return on plan assets	(57,000,000)	(57,000,000)	(170,000,000)	(172,000,000)
Amortization of prior service credit	0	0	0	0
Amortization of actuarial loss	(11,000,000)	(12,000,000)	(33,000,000)	(37,000,000)
Net periodic cost (benefit)	(5,000,000)	(8,000,000)	(15,000,000)	(25,000,000)
Other Postretirement Benefits Plan [Member]
Component of periodic benefit cost
Service cost	0	0	0	0
Interest cost	(2,000,000)	(1,000,000)	(6,000,000)	(4,000,000)
Expected return on plan assets	(1,000,000)	(2,000,000)	(3,000,000)	(5,000,000)
Amortization of prior service credit	(2,000,000)	(2,000,000)	(5,000,000)	(5,000,000)
Amortization of actuarial loss	(1,000,000)	(2,000,000)	(4,000,000)	(5,000,000)
Net periodic cost (benefit)	$ 0	$ (1,000,000)	$ 2,000,000	$ (1,000,000)