Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended			12 Months Ended
	Mar. 31, 2022 USD ($) loan credit_facility $ / shares	Dec. 31, 2021 USD ($)	Mar. 31, 2021 USD ($) $ / shares	Dec. 31, 2021 USD ($)	Jan. 01, 2022 USD ($)	Dec. 31, 2020 USD ($)	Apr. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage							85.00%
Restricted cash	$ 0	$ 0		$ 0
Borrower escrows	421,800,000	531,200,000		531,200,000
Increase (decrease) in CECL reserve	(2,500,000)		$ (1,400,000)
Reversal of remaining CECL reserve	2,537,000		1,293,000
CECL reserve	122,221,000	124,679,000	$ 172,100,000	124,679,000		$ 173,549,000
Principal balance	23,708,990,000	22,003,017,000		22,003,017,000
Decrease to additional paid-in capital	(5,431,627,000)	(5,373,029,000)		(5,373,029,000)
Decrease to accumulated deficit	$ (798,992,000)	(794,832,000)		(794,832,000)
Decrease to diluted earnings per share (in dollars per share) | $ / shares	$ (0.58)		$ (0.54)
Multifamily Asset in New York City
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in CECL reserve		14,800,000
Reversal of remaining CECL reserve		$ 360,000
CECL reserve	$ 0
Credit Spread Option | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan	37
Number of credit facilities | credit_facility	8
Average compounded SOFR (percentage)	0.30%
Credit Spread Option | United State Dollar LIBOR Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average compounded SOFR (percentage)	0.45%
Level 3 | Measurement Input, Cap Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0480
Level 3 | Measurement Input, Discount Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Measurement input (percentage)	0.0830
Level 3 | Fair Value, Nonrecurring
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount charged off				14,400,000
Increase (decrease) in CECL reserve				14,800,000
Reversal of remaining CECL reserve				$ 360,000
CECL reserve	$ 54,900,000
Principal balance	$ 286,300,000
Accounting Standards Update 2020-06 | Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease to additional paid-in capital					$ 2,400,000
Decrease to accumulated deficit					2,000,000.0
Increase to convertible notes, net					$ 476,000
Decrease to diluted earnings per share (in dollars per share) | $ / shares	$ 0.01
Walker and Dunlop
Accounts, Notes, Loans and Financing Receivable [Line Items]
Joint venture capital contribution percentage							15.00%