Nature of Operations and Significant Accounting Policies - Restructuring Charges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge	$ 0	$ 0	$ 240
Non-Operating Restructuring Charge	$ 0	$ 0	85
Management streamlining
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			240
Non-Operating Restructuring Charge			85
Management streamlining | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			121
Non-Operating Restructuring Charge			73
Management streamlining | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			98
Non-Operating Restructuring Charge			56
Management streamlining | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			0
Non-Operating Restructuring Charge			17
Management streamlining | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			23
Non-Operating Restructuring Charge			0
Management streamlining | Reimbursement Arrangements
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			84
Non-Operating Restructuring Charge			0
Management streamlining | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			24
Non-Operating Restructuring Charge			0
Management streamlining | Pension Settlement Charge
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			0
Non-Operating Restructuring Charge			12
Management streamlining | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge			11
Non-Operating Restructuring Charge			$ 0