Fair value measurements - Summary of financial instruments measured at fair value (Details) - USD ($)	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Assets:
Derivative assets	$ 18,499,000	$ 19,781,000
Cash equivalents	442,000	0
AG Arc	39,680,000	53,435,000
Total Assets Measured at Fair Value	4,230,937,000	3,222,792,000
Liabilities:
Securitized debt	(3,262,352,000)	(999,215,000)
Derivative liabilities	(9,000)	(976,000)
Total Liabilities Measured at Fair Value	$ (3,262,361,000)	$ (1,000,191,000)
Derivative Asset, Statement of Financial Position [Extensible Enumeration]	Other assets	Other assets
Residential Mortgage
Assets:
Residential mortgage loans	$ 421,451,000	$ 1,476,972,000
Residential Mortgage | Securitized residential mortgage loans
Assets:
Residential mortgage loans	3,707,146,000	1,158,134,000
Non-Agency RMBS
Assets:
Debt securities, available for sale	19,537,000	15,362,000
Non-Agency RMBS Interest Only
Assets:
Debt securities, available for sale	5,058,000	3,395,000
Agency Interest Only
Assets:
Debt securities, available for sale	19,124,000
30 Year Fixed Rate Agency RMBS
Assets:
Debt securities, available for sale		495,713,000
Interest Rate Swaps
Liabilities:
Derivative asset, reduction in fair value related to variation margin	17,300,000	19,600,000
Derivative liability, reduction in fair value related to variation margin		900,000
Level 1
Assets:
Derivative assets	0	0
Cash equivalents	442,000
AG Arc	0	0
Total Assets Measured at Fair Value	442,000	0
Liabilities:
Securitized debt	0	0
Derivative liabilities	0	0
Total Liabilities Measured at Fair Value	0	0
Level 1 | Residential Mortgage
Assets:
Residential mortgage loans	0	0
Level 1 | Residential Mortgage | Securitized residential mortgage loans
Assets:
Residential mortgage loans	0	0
Level 1 | Non-Agency RMBS
Assets:
Debt securities, available for sale	0	0
Level 1 | Non-Agency RMBS Interest Only
Assets:
Debt securities, available for sale	0	0
Level 1 | Agency Interest Only
Assets:
Debt securities, available for sale	0
Level 1 | 30 Year Fixed Rate Agency RMBS
Assets:
Debt securities, available for sale		0
Level 2
Assets:
Derivative assets	18,401,000	19,781,000
Cash equivalents	0
AG Arc	0	0
Total Assets Measured at Fair Value	47,957,000	516,409,000
Liabilities:
Securitized debt	0	0
Derivative liabilities	0	(897,000)
Total Liabilities Measured at Fair Value	0	(897,000)
Level 2 | Residential Mortgage
Assets:
Residential mortgage loans	754,000	915,000
Level 2 | Residential Mortgage | Securitized residential mortgage loans
Assets:
Residential mortgage loans	0	0
Level 2 | Non-Agency RMBS
Assets:
Debt securities, available for sale	9,678,000	0
Level 2 | Non-Agency RMBS Interest Only
Assets:
Debt securities, available for sale	0	0
Level 2 | Agency Interest Only
Assets:
Debt securities, available for sale	19,124,000
Level 2 | 30 Year Fixed Rate Agency RMBS
Assets:
Debt securities, available for sale		495,713,000
Level 3
Assets:
Derivative assets	98,000	0
Cash equivalents	0
AG Arc	39,680,000	53,435,000
Total Assets Measured at Fair Value	4,182,538,000	2,706,383,000
Liabilities:
Securitized debt	(3,262,352,000)	(999,215,000)
Derivative liabilities	(9,000)	(79,000)
Total Liabilities Measured at Fair Value	(3,262,361,000)	(999,294,000)
Level 3 | Residential Mortgage
Assets:
Residential mortgage loans	420,697,000	1,476,057,000
Level 3 | Residential Mortgage | Securitized residential mortgage loans
Assets:
Residential mortgage loans	3,707,146,000	1,158,134,000
Level 3 | Non-Agency RMBS
Assets:
Debt securities, available for sale	9,859,000	15,362,000
Level 3 | Non-Agency RMBS Interest Only
Assets:
Debt securities, available for sale	5,058,000	3,395,000
Level 3 | Agency Interest Only
Assets:
Debt securities, available for sale	$ 0
Level 3 | 30 Year Fixed Rate Agency RMBS
Assets:
Debt securities, available for sale		$ 0