Employee Retirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Qualified
Net periodic pension cost for the company's defined-benefit pension plans
Service cost	$ 1	$ 1	$ 2	$ 2
Interest cost	11	13	36	34
Expected return on plan assets	(12)	(15)	(36)	(34)
Amortization of net loss	5	5	15	12
Net periodic pension cost	5	4	17	14
Non-Qualified
Net periodic pension cost for the company's defined-benefit pension plans
Service cost	0	0	0	0
Interest cost	2	1	5	5
Expected return on plan assets	0	0	0	0
Amortization of net loss	1	1	2	2
Net periodic pension cost	$ 3	$ 2	$ 7	$ 7