SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Mortgage Banking Activities
Co-broker fees	$ 7.9	$ 5.4	$ 15.3	$ 8.2
Fair value of expected guaranty obligation recognized at commitment	$ 1.3	$ 4.4	$ 2.6	$ 8.5