Fair Value of Financial Instruments - Information about Significant Unobservable Inputs used in Fair Value Measurement (Details) - Level III	12 Months Ended
Dec. 31, 2017
Equity securities in private companies \| Investments at fair value \| Minimum \| Market approach
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Revenue multiple	1.9
EBITDA multiple	10.6
Equity securities in private companies \| Investments at fair value \| Maximum \| Market approach
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Revenue multiple	6.2
EBITDA multiple	15
Equity securities in private companies \| Investments at fair value \| Weighted Average \| Market approach
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Revenue multiple	4.9
EBITDA multiple	12.8
Interest rate locks \| Financial instruments and other inventory positions sold, but not yet purchased \| Minimum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Premium over the MMD curve	1
Interest rate locks \| Financial instruments and other inventory positions sold, but not yet purchased \| Maximum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Premium over the MMD curve	15.92
Interest rate locks \| Financial instruments and other inventory positions sold, but not yet purchased \| Weighted Average \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Premium over the MMD curve	7.91
Tax-exempt securities \| Financial instruments and other inventory positions owned \| Minimum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)	5.00%
Tax-exempt securities \| Financial instruments and other inventory positions owned \| Maximum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)	60.00%
Tax-exempt securities \| Financial instruments and other inventory positions owned \| Weighted Average \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)	19.40%
Short-term securities \| Financial instruments and other inventory positions owned \| Minimum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)	66.00%
Short-term securities \| Financial instruments and other inventory positions owned \| Maximum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)	94.00%
Short-term securities \| Financial instruments and other inventory positions owned \| Weighted Average \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Expected recovery rate (% of par)	91.00%
Collateralized by residential mortgages \| Financial instruments and other inventory positions owned \| Minimum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Credit default rate	1.00%
Prepayment rates	20.00%
Loss severity	25.00%
Valuation yields	5.00%
Collateralized by residential mortgages \| Financial instruments and other inventory positions owned \| Maximum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Credit default rate	2.00%
Prepayment rates	0.00%
Loss severity	50.00%
Valuation yields	0.00%
Collateralized by residential mortgages \| Financial instruments and other inventory positions owned \| Weighted Average \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Credit default rate	1.50%
Prepayment rates	20.00%
Loss severity	26.90%
Valuation yields	5.00%
Interest rate locks \| Financial instruments and other inventory positions owned \| Minimum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Premium over the MMD curve	7.42
Interest rate locks \| Financial instruments and other inventory positions owned \| Maximum \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Premium over the MMD curve	15.26
Interest rate locks \| Financial instruments and other inventory positions owned \| Weighted Average \| Discounted cash flow
Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]
Premium over the MMD curve	11.08

Fair Value Inputs Assets and Liabilities Quantitative Information [Line Items]

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- Definition

Expected recovery rate (% of par), which is one of the factors applied in estimating the fair value of financial assets and liabilities.

+ References

No definition available.

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- Definition

The unamortized premium over the MMD curve used to value derivative contracts.

+ References

No definition available.

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- Definition

Estimated valuation yields, which may be one of the factors applied in estimating the fair value of financial assets and liabilities.

+ References

No definition available.

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- Definition

Market valuation of entity divided by earnings before interest, taxes, depreciation, and amortization (EBITDA), used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

+ References

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- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ References

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Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

+ References

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- Definition

Market valuation of entity divided by revenue, used as an input to measure fair value.

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