Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plan [Member]
Component of periodic benefit cost
Service cost	$ 1	$ 1
Interest cost	(35)	(49)
Expected return on plan assets	(56)	(79)
Defined Benefit Plan, Amortization of Gain (Loss)	(12)	(15)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	(8)	(14)
Other Postretirement Benefits Plan [Member]
Component of periodic benefit cost
Service cost	0	0
Interest cost	(2)	(2)
Expected return on plan assets	(2)	(2)
Amortization of prior service credit	(1)	(1)
Defined Benefit Plan, Amortization of Gain (Loss)	(1)	(1)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment	$ 0	$ 0