INVESTMENT SECURITIES - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) security	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Mar. 01, 2019 USD ($)
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0		$ 0
Securities pledged as collateral	1,200,000,000		1,200,000,000		$ 914,500,000
Sale of investment securities available for sale, including interest			602,500,000
Remaining securities			23,700,000
Debt securities, available-for-sale, realized gain (less than)			100,000	$ 100,000
Losses from sale of available-for-sale securities			0	0
Sale of investment securities available for sale			602,467,000	7,012,000
Proceeds from sale of equity securities			0	6,200,000
Realized gain on sale of equity investment	0	$ 3,757,000	0	3,757,000
Realized loss on sale of equity investment				$ 0
Unamortized premiums			16,700,000		12,700,000
Unaccreted discounts			3,500,000		2,500,000
Owned investment securities	217,200,000		217,200,000
Total unrealized losses on securities	$ 1,500,000		$ 1,500,000
Number of securities, below investment grade | security	1		1
Debt securities, below investment grade, fair value	$ 400,000		$ 400,000
Other than temporary impairment loss on debt securities			$ 0		$ 0
Weighted average duration of MBS portfolio			2 years 10 months 24 days
Beneficial
Gain (Loss) on Securities [Line Items]
Securities sold						$ 578,800,000