Summary of Significant Accounting Policies - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) segment	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
New Accounting Pronouncements or Change in Accounting Principle
Provision (reversal) for expected credit losses	$ 3,000	$ 800	$ 1,700	$ 1,600
Guaranty liabilities	26,889		26,889		$ 28,551	$ 27,729
Allowance for doubtful accounts, period increase (decrease)	19	(500)	(1,800)	600
Allowance for doubtful accounts	17,700		17,700		19,500
Allowance for credit loss	9,400		9,400		$ 9,400
Revenues	685,912	616,283	$ 1,865,786	1,722,926
Number of operating segments | segment			1
Capital markets
New Accounting Pronouncements or Change in Accounting Principle
Revenues	$ 188,708	159,266	$ 481,514	416,040
Capital markets | Revision of Prior Period, Reclassification, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Revenues		159,300		416,000
Investment sales | Revision of Prior Period, Reclassification, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Revenues		(94,700)		(244,300)
Commercial origination, net of investment sales | Revision of Prior Period, Reclassification, Adjustment
New Accounting Pronouncements or Change in Accounting Principle
Revenues		$ (64,600)		$ (171,700)