Fair Value of Financial Instruments - Information about Significant Unobservable Inputs used in Fair Value Measurement (Details) - Level III	Dec. 31, 2020 basis_points
Interest rate locks | Financial instruments and other inventory positions sold, but not yet purchased | Discounted cash flow | Premium over the MMD curve | Minimum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Derivative contracts, liabilities	0
Interest rate locks | Financial instruments and other inventory positions sold, but not yet purchased | Discounted cash flow | Premium over the MMD curve | Maximum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Derivative contracts, liabilities	8
Interest rate locks | Financial instruments and other inventory positions sold, but not yet purchased | Discounted cash flow | Premium over the MMD curve | Weighted Average
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Derivative contracts, liabilities	2.4
Financial instruments and other inventory positions owned | Interest rate locks | Discounted cash flow | Premium over the MMD curve | Minimum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Derivative contracts, assets	1
Financial instruments and other inventory positions owned | Interest rate locks | Discounted cash flow | Premium over the MMD curve | Maximum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Derivative contracts, assets	3
Financial instruments and other inventory positions owned | Interest rate locks | Discounted cash flow | Premium over the MMD curve | Weighted Average
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Derivative contracts, assets	1.8
Investments at fair value | Equity securities in private companies | Market approach | Revenue multiple | Minimum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Investments at fair value	3
Investments at fair value | Equity securities in private companies | Market approach | Revenue multiple | Maximum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Investments at fair value	5
Investments at fair value | Equity securities in private companies | Market approach | Revenue multiple | Weighted Average
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Investments at fair value	4.1
Investments at fair value | Equity securities in private companies | Market approach | EBITDA multiple | Minimum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Investments at fair value	9
Investments at fair value | Equity securities in private companies | Market approach | EBITDA multiple | Maximum
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Investments at fair value	20
Investments at fair value | Equity securities in private companies | Market approach | EBITDA multiple | Weighted Average
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Investments at fair value	15.8