Employee Retirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Qualified
Net periodic pension cost for the company's defined-benefit pension plans
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	8	11	28	36
Expected return on plan assets	(12)	(12)	(36)	(36)
Amortization of net loss	4	5	13	15
Net periodic pension cost	1	5	7	17
Non-Qualified
Net periodic pension cost for the company's defined-benefit pension plans
Service cost	0	0	0	0
Interest cost	1	2	4	5
Expected return on plan assets	0	0	0	0
Amortization of net loss	1	1	2	2
Net periodic pension cost	$ 2	$ 3	$ 6	$ 7