Pension and Postretirement Benefit Plans - Narrative (Details) - Funded $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Qualified and Non-qualified Pension Benefits
Benefit Plan Information
Company contributions year to date	$ 231
Qualified and Non-qualified Pension Benefits | Maximum
Benefit Plan Information
Estimated pension and postretirement employer contributions in current fiscal year	500
Qualified and Non-qualified Pension Benefits | Minimum
Benefit Plan Information
Estimated pension and postretirement employer contributions in current fiscal year	300
Postretirement Benefits
Benefit Plan Information
Company contributions year to date	$ 1