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Number of limited partnership units granted exchangeability into the company's class A common stock during the period.

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Amount of cash inflow (outflow) of investing activities of discontinued operations. Investing activity cash flows include making and collecting loans and acquiring and disposing of debt or equity instruments and property, plant, and equipment and other productive assets.

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Amount of cash inflow (outflow) of operating activities of discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount of equity impact of cash distribution declared to unit-holder of limited partnership (LP).

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Aggregate amount of interest paid or due on all long-term debt.

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

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Discontinued Operations - Additional Information (Detail) - USD ($) shares in Millions				3 Months Ended
	Nov. 23, 2018	Nov. 07, 2018	Sep. 05, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Mar. 19, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash used in operating activities from discontinued operations					$ (485,477,000)
Net cash provided by investing activities from discontinued operations					40,822,000
Equity-based compensation expense				$ 1,100,000
Income allocated to limited partnership units				4,500,000	3,800,000
Revolving Credit Facility [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings		$ 150,000,000
Intercompany Credit Agreement [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Line of credit facility, outstanding amount							$ 150,000,000
Cantor [Member] \| Revolving Credit Facility [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Line of credit facility, outstanding amount				$ 0			$ 150,000,000
8.125% Senior Notes Due 2042 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Stated interest rate			8.125%	8.125%		8.125%
Debt maturity year			2042	2042
Repayments of outstanding borrowings			$ 112,500,000
Interest expense recorded				$ 0	0
5.375% Senior Notes Due 2019 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Stated interest rate	5.375%			5.375%		5.375%
Debt maturity year	2019			2019
Repayments of outstanding borrowings	$ 300,000,000
Interest expense recorded				$ 0	$ 0
Newmark [Member] \| Intercompany Credit Agreement [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings		$ 150,000,000
Newmark [Member] \| 8.125% Senior Notes Due 2042 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings			$ 112,500,000
Newmark [Member] \| 5.375% Senior Notes Due 2019 [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Repayments of outstanding borrowings	$ 300,000,000
Limited Partnership Units [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of limited partnership units granted exchangeability					1.5
Equity-based compensation expense				$ 5,000,000	$ 34,500,000
Limited Partnership Units [Member] \| Newmark [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of limited partnership units granted exchangeability				0.1	1.4
Newmark [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Net cash used in operating activities from discontinued operations					$ 485,500,000
Net cash provided by investing activities from discontinued operations					40,800,000
Income allocated to limited partnership units					5,300,000
Interest expense recorded					13,800,000
Newmark [Member] \| Intercompany Credit Agreement [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Interest expense recorded					$ 300,000
Newmark [Member] \| Limited Partnership Units [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Number of limited partnership units granted exchangeability					0.7
Equity-based compensation expense					$ 21,700,000