Nature of Operations and Significant Accounting Policies - Restructuring Charges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 30, 2016
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge	$ 0	$ 240	$ 0
Non-Operating Restructuring Charge	$ 0	85	0
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
As Previously Reported
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		325	0
Non-Operating Restructuring Charge		0	$ 0
As Previously Reported | Management streamlining
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		325
As Previously Reported | Management streamlining | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		194
As Previously Reported | Management streamlining | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		154
As Previously Reported | Management streamlining | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		17
As Previously Reported | Management streamlining | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		23
As Previously Reported | Management streamlining | Reimbursement Arrangements
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		84
As Previously Reported | Management streamlining | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		24
As Previously Reported | Management streamlining | Pension Settlement Charge
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		12
As Previously Reported | Management streamlining | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Operating Restructuring Charge		$ 11