Employee Benefit Plans - Components of Net Periodic Benefit Cost (Benefit) and Other Amounts Recognized in Other Comprehensive Income (Loss), Additional Information (Details) - USD ($)	1 Months Ended	6 Months Ended		12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Reduction in interest cost component of net periodic interest						$ 32
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pre-tax settlement charge				$ 0	$ 0	750,000,000
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlements, Net of Tax						(488,000,000)
Reduction to stockholder's equity						144,000,000
Employer contributions [1]				70,000,000	103,000,000
Increase to unrecognized net loss from pension settlement transaction						$ 750,000,000
Pension Benefits | Qualified Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions [1]	$ 280,000,000
Discount rate used to measure interest cost		3.37%	3.58%
Discount rate used to measure interest cost, historical method		3.92%	4.22%
Pension Benefits | Non-Qualified Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate used to measure interest cost		3.55%
Discount rate used to measure interest cost, historical method		4.19%
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Pre-tax settlement charge				0
Employer contributions [1]				$ 0	$ 0
Discount rate used to measure interest cost		3.13%
Discount rate used to measure interest cost, historical method		3.97%