Interim Financial Data - Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2020	Apr. 01, 2020	Mar. 31, 2020
Summary Of Significant Accounting Policies [Line Items]
Retained earnings	$ 14,382		$ 13,571
Allowance for credit losses	$ 1,137		$ 727
ASU 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Apr. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	false
ASU 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Apr. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2018-15 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Apr. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2019-04 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Apr. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
ASU 2018-17 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adoption date	Apr. 01, 2020
Change in accounting principle, accounting standards update, adopted [true false]	true
Change in accounting principle, accounting standards update, immaterial effect [true false]	true
Cumulative Effect Period of Adoption Adjustment [Member] | ASU 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained earnings		$ 218
Allowance for credit losses		292
Accrued interest		$ 190