Retirement Plans and Other Postretirement Benefits - Narrative (Details)	6 Months Ended
	Jun. 30, 2019 USD ($)
Pension Benefits
Contributions
Voluntary employer contributions to pension plan	$ 120,000,000
Minimum employer contributions for the next three years	0
Maximum employer contributions for the next two years (up to)	350,000,000
Other Benefits
Contributions
Estimated future employer contributions in next three years	$ 0