Employee Benefit Plans - Narrative (Details)		1 Months Ended	3 Months Ended	12 Months Ended
	Aug. 29, 2017 USD ($) participant	Sep. 30, 2018 USD ($) multiemployer_plan participant	Dec. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) multiemployer_plan	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Oct. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Remeasurement gain (loss)			$ 8,000,000	$ (38,000,000)	$ 68,000,000	$ 42,000,000
Contribution plan expense				$ 14,000,000	14,000,000	11,000,000
Number of multiemployer plans | multiemployer_plan		2		2
Estimated partial withdrawal liability								$ 30,000,000
Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Potential settlement amount							$ 10,000,000
Domestic Plan | Fixed securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations		75.00%		75.00%
Domestic Plan | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocations		25.00%		25.00%
Pension benefits
Defined Benefit Plan Disclosure [Line Items]
Remeasurement gain (loss)				$ 95,000,000	60,000,000
Accumulated benefit obligations		$ 2,087,000,000		2,087,000,000	2,381,000,000	3,138,000,000
Employer contributions				16,000,000	412,000,000
Pension benefits | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Number of participants electing to receive lump sum payment settlement | participant		2,600
Lump sum payments		$ 134,000,000
Number of participants | participant	6,000
Outstanding benefit obligation transfered in exchange for pension trust assets	$ 585,000,000
Pension benefits | UNITED STATES | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					394,000,000
Expected future contribution		0		0
Pension benefits | UNITED STATES | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Expected future contribution		$ 13,000,000		$ 13,000,000
Other postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Annual base year per capita costs, basis spread, up to				1.50%
Assumed pre-65 health care cost trend rate		7.50%		7.50%
Ultimate pre-65 health care cost trend rate		4.50%		4.50%
Remeasurement gain (loss)				$ 0	5,000,000
Accumulated benefit obligations		$ 51,000,000		51,000,000	57,000,000	$ 73,000,000
Employer contributions				7,000,000	13,000,000
Other various benefit plans
Defined Benefit Plan Disclosure [Line Items]
Current postemployment benefits liability		1,000,000		1,000,000	1,000,000
Noncurrent postemployment benefits liability		$ 3,000,000		$ 3,000,000	$ 4,000,000