Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019		Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Impairments of equity securities related to value changes resulting from observable prices
Stockholders' equity		48,039	44,699	$ 47,371	$ 45,344		$ 43,818	$ 41,035
Realized gains (losses) on investments still held at the reporting date
Fair value of collateral posted
Gain (loss) recognized in the income statement
Gain (loss) recognized in other comprehensive income
Gains (losses) reclassified from other comprehensive income into shareholders' net income
Amounts excluded from assessment of hedge effectiveness
Retained Earnings
Schedule of Investments [Line Items]
Stockholders' equity		24,440	$ 19,185	$ 23,052	20,162		$ 17,834	15,088
Adjustment upon Adoption
Schedule of Investments [Line Items]
Stockholders' equity					(30)	[1]		(15)
Allowance for credit losses on financing receivables					7
Adjustment upon Adoption | Retained Earnings
Schedule of Investments [Line Items]
Stockholders' equity					(30)	[1]		$ (15)
Commercial Portfolio Segment | Real Estate Loan
Schedule of Investments [Line Items]
Allowance for credit losses on financing receivables		9
Current loans		$ 1,979			1,947
Commercial Portfolio Segment | Real Estate Loan | Adjustment upon Adoption | Retained Earnings
Schedule of Investments [Line Items]
Stockholders' equity					$ (7)

[1] See Note 3 for further information about the Company's adoption of new credit loss guidance (ASU 2016-13).