Summary of Significant Accounting Policies - Accounting standards (Details) - USD ($)		12 Months Ended
	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Jan. 01, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Present value of lease liabilities		$ 586,000,000
Right of use asset		$ 483,000,000
Expected returns on plan assets, smoothing period		5 years
Total shareholders’ equity		$ 25,998,000,000		$ 21,312,000,000		$ 22,551,000,000
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Present value of lease liabilities			$ 585,000,000
Right of use asset			488,000,000
Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle			21,000,000		$ 1,088,000,000				$ 0
Total shareholders’ equity		48,074,000,000		44,033,000,000		41,579,000,000		$ 39,009,000,000
Retained income | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle			21,000,000
Forecast | Retained income | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle	$ (100,000,000)
Allowance for credit loss, increase (decrease)	$ 300,000,000
Change in accounting principle | Change in accounting principle
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total shareholders’ equity		(158,000,000)	(41,000,000)	0	0
Change in accounting principle | Change in accounting principle | Retained income
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of change in accounting principle			$ 0		$ 0
Total shareholders’ equity		$ (1,639,000,000)		$ (1,675,000,000)		$ (1,583,000,000)	$ (1,580,000,000)	$ (1,669,000,000)