Nature of Operations and Significant Accounting Policies - Restructuring Charges (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring Charge	$ 0	$ 115	$ 0	$ 225
Restructuring Charges Nonoperating	$ 0	7	$ 0	70
Management Streamlining Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		115		225
Restructuring Charges Nonoperating		7		70
Management Streamlining Initiative | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		15		107
Restructuring Charges Nonoperating		7		70
Management Streamlining Initiative | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		10		91
Restructuring Charges Nonoperating		3		53
Management Streamlining Initiative | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		0		0
Restructuring Charges Nonoperating		4		17
Management Streamlining Initiative | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		5		16
Restructuring Charges Nonoperating		0		0
Management Streamlining Initiative | Reimbursement Arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		84		84
Restructuring Charges Nonoperating		0		0
Management Streamlining Initiative | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		16		24
Restructuring Charges Nonoperating		0		0
Management Streamlining Initiative | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				10
Restructuring Charges Nonoperating				0
As Previously Reported
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		122		295
Restructuring Charges Nonoperating		0		0
As Previously Reported | Management Streamlining Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		122		295
As Previously Reported | Management Streamlining Initiative | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		22		177
As Previously Reported | Management Streamlining Initiative | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		13		144
As Previously Reported | Management Streamlining Initiative | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		4		17
As Previously Reported | Management Streamlining Initiative | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		5		16
As Previously Reported | Management Streamlining Initiative | Reimbursement Arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		84		84
As Previously Reported | Management Streamlining Initiative | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		$ 16		24
As Previously Reported | Management Streamlining Initiative | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Restructuring Charge				$ 10