Nature of Operations and Significant Accounting Policies - Restructuring Charges (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring Charge	$ 0	$ 1	$ 0	$ 226
Restructuring Charges Nonoperating	$ 0	0	$ 0	70
Management Streamlining Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				226
Restructuring Charges Nonoperating				70
Management Streamlining Initiative | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				107
Restructuring Charges Nonoperating				70
Management Streamlining Initiative | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				91
Restructuring Charges Nonoperating				53
Management Streamlining Initiative | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				0
Restructuring Charges Nonoperating				17
Management Streamlining Initiative | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				16
Restructuring Charges Nonoperating				0
Management Streamlining Initiative | Reimbursement Arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				84
Restructuring Charges Nonoperating				0
Management Streamlining Initiative | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				24
Restructuring Charges Nonoperating				0
Management Streamlining Initiative | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				11
Restructuring Charges Nonoperating				0
As Previously Reported
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		1		296
Restructuring Charges Nonoperating		$ 0		0
As Previously Reported | Management Streamlining Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				296
As Previously Reported | Management Streamlining Initiative | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				177
As Previously Reported | Management Streamlining Initiative | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				144
As Previously Reported | Management Streamlining Initiative | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				17
As Previously Reported | Management Streamlining Initiative | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				16
As Previously Reported | Management Streamlining Initiative | Reimbursement Arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				84
As Previously Reported | Management Streamlining Initiative | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				24
As Previously Reported | Management Streamlining Initiative | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				$ 11