Significant Accounting Policies - Variable Interest Entities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Variable Interest Entity [Line Items]
Long-term investments	$ 17,314	$ 15,732
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Long-term investments	1,021	1,069
Hedge fund investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Long-term investments	271	270
Private equity investments | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Long-term investments	538	524
Real estate partnerships | Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Long-term investments	$ 212	$ 275