Significant Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended	6 Months Ended
	Sep. 30, 2020 USD ($) segment	Dec. 31, 2020 USD ($) segment	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of reportable segments | segment	4	4
Shareholders' equity	$ 6,090	$ (277)	$ 5,309	$ 6,385	$ 6,692	$ 8,287
Allowance for credit losses		221
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity	$ 13,890	$ 7,595	13,022	$ 12,075	$ 11,965	12,409
Opening retained earnings adjustment: adoption of new accounting standard
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity			(13)			11
Opening retained earnings adjustment: adoption of new accounting standard | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity			(13)			$ 11
ASU 2016-13 | Opening retained earnings adjustment: adoption of new accounting standard | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Shareholders' equity			$ 13