Nature of Operations and Significant Accounting Policies - Restructuring Charges (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring Charge	$ 0	$ 110
Restructuring Charges Nonoperating	$ 0	63
Management Streamlining Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		110
Restructuring Charges Nonoperating		63
Management Streamlining Initiative | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		92
Restructuring Charges Nonoperating		63
Management Streamlining Initiative | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		81
Restructuring Charges Nonoperating		50
Management Streamlining Initiative | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		0
Restructuring Charges Nonoperating		13
Management Streamlining Initiative | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		11
Restructuring Charges Nonoperating		0
Management Streamlining Initiative | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		8
Restructuring Charges Nonoperating		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		173
Restructuring Charges Nonoperating		0
As Previously Reported | Management Streamlining Initiative
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		173
As Previously Reported | Management Streamlining Initiative | Subtotal Management Workforce Reduction
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		155
As Previously Reported | Management Streamlining Initiative | Severance and Pension
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		131
As Previously Reported | Management Streamlining Initiative | Other Post-Retirement Benefits Curtailment
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		13
As Previously Reported | Management Streamlining Initiative | Employee Equity Awards Proration and Other
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		11
As Previously Reported | Management Streamlining Initiative | Executive Equity Awards Proration
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		8
As Previously Reported | Management Streamlining Initiative | Advisory Fees Related to Shareholder Matters
Restructuring Cost and Reserve [Line Items]
Restructuring Charge		$ 10