INVESTMENT SECURITIES - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Investment Securities
Available-for-sale:
Amount Outstanding	$ 5,229,446	$ 5,120,963
Amortized cost	5,165,562	5,060,135
Allowance for losses	(25)	(27)
Unrealized Gains	5,441	9,844
Unrealized Losses	(163,191)	(151,021)
Fair Value	5,007,787	4,918,931
Held-to-maturity:
Amount Outstanding	53,756	53,756
Amortized Cost	53,756	53,756
Allowance for losses	0	0
Unrealized Gains	1,104	1,745
Unrealized Losses	0	0
Fair Value	54,860	55,501
Available-for-sale, accrued interest excluded	22,200	15,900
Available-for-Sale Securities \| Investment Securities
Available-for-sale:
Unamortized Premium/(Discount)	(63,884)	(60,828)
Held-to-maturity:
Unamortized Premium/(Discount)	(63,884)	(60,828)
Held-to-Maturity Securities \| Investment Securities
Available-for-sale:
Unamortized Premium/(Discount)	0	0
Held-to-maturity:
Unamortized Premium/(Discount)	0	0
Floating interest rate \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Available-for-sale:
Amount Outstanding	19,700	19,700
Amortized cost	19,700	19,700
Allowance for losses	(25)	(27)
Unrealized Gains	0	0
Unrealized Losses	(394)	(591)
Fair Value	19,281	19,082
Floating interest rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,402,850	2,454,009
Amortized cost	2,401,544	2,452,871
Allowance for losses	0	0
Unrealized Gains	2,608	1,212
Unrealized Losses	(24,086)	(29,649)
Fair Value	2,380,066	2,424,434
Floating interest rate \| US Treasuries
Available-for-sale:
Amount Outstanding	50,000	50,000
Amortized cost	49,990	49,983
Allowance for losses	0	0
Unrealized Gains	6	0
Unrealized Losses	0	(15)
Fair Value	49,996	49,968
Floating interest rate \| Available-for-Sale Securities \| Floating rate auction-rate certificates backed by Government guaranteed student loans
Available-for-sale:
Unamortized Premium/(Discount)	0	0
Held-to-maturity:
Unamortized Premium/(Discount)	0	0
Floating interest rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	(1,306)	(1,138)
Held-to-maturity:
Unamortized Premium/(Discount)	(1,306)	(1,138)
Floating interest rate \| Available-for-Sale Securities \| US Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	(10)	(17)
Held-to-maturity:
Unamortized Premium/(Discount)	(10)	(17)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	1,851,641	1,727,669
Amortized cost	1,804,193	1,680,881
Allowance for losses	0	0
Unrealized Gains	2,746	6,558
Unrealized Losses	(134,551)	(117,824)
Fair Value	1,672,388	1,569,615
Held-to-maturity:
Amount Outstanding	53,756	53,756
Amortized Cost	53,756	53,756
Allowance for losses	0	0
Unrealized Gains	1,104	1,745
Unrealized Losses	0	0
Fair Value	$ 54,860	$ 55,501
Held-to-maturity investment securities weighted average yield	6.70%	6.70%
Fixed Interest Rate \| US Treasuries
Available-for-sale:
Amount Outstanding	$ 905,255	$ 869,585
Amortized cost	890,135	856,700
Allowance for losses	0	0
Unrealized Gains	81	2,074
Unrealized Losses	(4,160)	(2,942)
Fair Value	886,056	855,832
Fixed Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	(47,448)	(46,788)
Held-to-maturity:
Unamortized Premium/(Discount)	(47,448)	(46,788)
Fixed Interest Rate \| Available-for-Sale Securities \| US Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	(15,120)	(12,885)
Held-to-maturity:
Unamortized Premium/(Discount)	(15,120)	(12,885)
Fixed Interest Rate \| Held-to-Maturity Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	0	0
Held-to-maturity:
Unamortized Premium/(Discount)	$ 0	$ 0

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