Employee Retirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Qualified
Net periodic pension cost for the company's defined-benefit pension plans
Service cost	$ 0	$ 0	$ 1	$ 1
Interest cost	10	13	20	25
Expected return on plan assets	(12)	(12)	(24)	(24)
Amortization of net loss	5	5	9	10
Net periodic pension cost	3	6	6	12
Non-Qualified
Net periodic pension cost for the company's defined-benefit pension plans
Service cost	0	0	0	0
Interest cost	2	2	3	3
Expected return on plan assets	0	0	0	0
Amortization of net loss	0	0	1	1
Net periodic pension cost	$ 2	$ 2	$ 4	$ 4