Significant Accounting Policies - Variable Interest Entities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Long-term investments	$ 20,812	$ 17,314
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Long-term investments	1,089	1,021
Variable Interest Entity, Not Primary Beneficiary | Hedge fund investments
Variable Interest Entity [Line Items]
Long-term investments	342	271
Variable Interest Entity, Not Primary Beneficiary | Private equity investments
Variable Interest Entity [Line Items]
Long-term investments	547	538
Variable Interest Entity, Not Primary Beneficiary | Real estate partnerships
Variable Interest Entity [Line Items]
Long-term investments	$ 200	$ 212