INVESTMENTS - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	$ 1,771,845	$ 579,528
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	63,701	2,076
Available-for-sale debt securities, 12 months or longer, Fair Value	4,330	4,355
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	3	3
Available-for-sale debt securities, Total, Fair Value	1,776,175	583,883
Available-for-sale debt securities, Total, Unrealized Loss	63,704	2,079
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	500
Held-to-maturity, Less than 12 months, Unrealized Loss	1
Held-to-maturity, 12 months or longer, Fair Value	0
Held-to-maturity, 12 months or longer, Unrealized Loss	0
Held-to-maturity, Total, Fair Value	500
Held-to-maturity, Total, Unrealized Loss	1
Foreign bonds
Held-to-maturity debt securities:
Held-to-maturity, Less than 12 months, Fair Value	500
Held-to-maturity, Less than 12 months, Unrealized Loss	1
Held-to-maturity, 12 months or longer, Fair Value	0
Held-to-maturity, 12 months or longer, Unrealized Loss	0
Held-to-maturity, Total, Fair Value	500
Held-to-maturity, Total, Unrealized Loss	1
CMO
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	288,625	183,983
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	15,223	443
Available-for-sale debt securities, 12 months or longer, Fair Value	0	0
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	0	0
Available-for-sale debt securities, Total, Fair Value	288,625	183,983
Available-for-sale debt securities, Total, Unrealized Loss	15,223	443
FNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	1,241,199	289,338
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	34,243	879
Available-for-sale debt securities, 12 months or longer, Fair Value	4,330	4,355
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	3	3
Available-for-sale debt securities, Total, Fair Value	1,245,529	293,693
Available-for-sale debt securities, Total, Unrealized Loss	34,246	882
FHLMC MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	17,936	5,191
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	759	105
Available-for-sale debt securities, 12 months or longer, Fair Value	0	0
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	0	0
Available-for-sale debt securities, Total, Fair Value	17,936	5,191
Available-for-sale debt securities, Total, Unrealized Loss	759	105
GNMA MBS
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	4,969
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	83
Available-for-sale debt securities, 12 months or longer, Fair Value	0
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	0
Available-for-sale debt securities, Total, Fair Value	4,969
Available-for-sale debt securities, Total, Unrealized Loss	83
GSE agency notes
Available-for-sale debt securities:
Available-for-sale debt securities, Less than 12 months, Fair Value	219,116	101,016
Available-for-sale debt securities, Less than 12 months, Unrealized Loss	13,393	649
Available-for-sale debt securities, 12 months or longer, Fair Value	0	0
Available-for-sale debt securities, 12 months or longer, Unrealized Loss	0	0
Available-for-sale debt securities, Total, Fair Value	219,116	101,016
Available-for-sale debt securities, Total, Unrealized Loss	$ 13,393	$ 649

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