ALLOWANCE FOR RISK-SHARING OBLIGATIONS AND GUARANTY OBLIGATION - CECL Provision Impact (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)
ALLOWANCE FOR RISK-SHARING OBLIGATIONS
Forecast-period loss rate	0.021	0.023	0.023	0.023	0.023	0.023
Reversion-period loss rate	0.012	0.013	0.013	0.015	0.015	0.015
Historical loss rate	0.003	0.003	0.003	0.006	0.006	0.006
At-risk Fannie Mae servicing portfolio UPB	$ 60,600.0	$ 59,500.0	$ 59,200.0	$ 57,400.0	$ 55,700.0	$ 54,500.0
CECL allowance	23.4	24.9	25.0	31.0	28.9	28.7
Provision (benefit) for CECL allowance	$ (1.5)	$ (0.1)	$ (1.5)	$ 0.5	$ 0.2	$ (11.0)	$ (3.1)	$ (10.3)