Pension and Other Postretirement Benefits - Additional Information (Details)				1 Months Ended		3 Months Ended		6 Months Ended
	Apr. 13, 2018 employee	Apr. 01, 2018 USD ($)	Nov. 01, 2016 USD ($) Installments	Mar. 31, 2018 USD ($)	Apr. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Change in unrecognized net actuarial loss and prior service cost/benefit related to pension and other postretirement benefits						$ 29,000,000	$ 48,000,000	$ 172,000,000	$ 79,000,000
Curtailments						9,000,000
Cash contribution term to pension plan			30 months
Number of installments | Installments			3
Minimum required cash contribution to pension plan			$ 50,000,000
Aggregate cash cash contribution to pension plan			$ 150,000,000
Cash payments made for pension and other post-retirement benefits				$ 50,000,000	$ 50,000,000
Labor agreement, duration	5 years
Labor agreement, number of employees | employee	1,300
Defined benefit plan, labor agreement, employees subject to retirement benefit increase | employee	300
Pension benefits
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Curtailments						$ 9,000,000	$ 0	$ 14,000,000	$ 0
UNITED STATES | Pension benefits
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Decrease in obligation, pension benefits		$ 136,000,000
Change in unrecognized net actuarial loss and prior service cost/benefit related to pension and other postretirement benefits		141,000,000
Curtailments		$ 5,000,000