Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Investment securities:
Available-for-sale:
Amount Outstanding	$ 5,177,029	$ 4,806,727
Unamortized Premium/(Discount)	0	0
Amortized cost	5,114,476	4,769,426
Allowance for losses	(27)	(33)
Unrealized Gains	2,088	4,703
Unrealized Losses	(243,123)	(194,532)
Fair Value	4,873,414	4,579,564
Held-to-maturity:
Amount Outstanding	45,032	45,032
Unamortized Premium/(Discount)	0	0
Amortized Cost	45,032	45,032
Allowance for losses	0	0
Unrealized Gains	930	2,433
Unrealized Losses	0	0
Fair Value	45,962	47,465
Available-for-sale, accrued interest excluded	17,400	10,600
Available-for-Sale Securities \| Investment securities:
Available-for-sale:
Unamortized Premium/(Discount)	(62,553)	(37,301)
Held-to-maturity:
Unamortized Premium/(Discount)	(62,553)	(37,301)
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	(27)	(33)
Unrealized Gains	0	0
Unrealized Losses	(640)	(640)
Fair Value	19,033	19,027
Floating Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,499,578	2,433,696
Amortized cost	2,498,408	2,433,496
Allowance for losses	0	0
Unrealized Gains	2,054	1,954
Unrealized Losses	(34,917)	(42,910)
Fair Value	2,465,545	2,392,540
Floating Interest Rate \| Fixed rate U.S. Treasuries
Available-for-sale:
Amount Outstanding	50,000
Amortized cost	49,977
Allowance for losses	0
Unrealized Gains	34
Unrealized Losses	0
Fair Value	50,011
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)	(1,170)	(200)
Held-to-maturity:
Unamortized Premium/(Discount)	(1,170)	(200)
Floating Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	(23)
Held-to-maturity:
Unamortized Premium/(Discount)	(23)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	1,629,166	1,207,416
Amortized cost	1,584,048	1,177,095
Allowance for losses	0	0
Unrealized Gains	0	2,128
Unrealized Losses	(197,670)	(130,837)
Fair Value	1,386,378	1,048,386
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Allowance for losses	0	0
Unrealized Gains	930	2,433
Unrealized Losses	0	0
Fair Value	$ 45,962	$ 47,465
Held-to-maturity investment securities weighted average yield	6.50%	4.50%
Fixed Interest Rate \| Fixed rate U.S. Treasuries
Available-for-sale:
Amount Outstanding	$ 978,585	$ 1,145,915
Amortized cost	962,343	1,139,135
Allowance for losses	0	0
Unrealized Gains	0	621
Unrealized Losses	(9,896)	(20,145)
Fair Value	952,447	1,119,611
Fixed Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	(45,118)	(30,321)
Held-to-maturity:
Unamortized Premium/(Discount)	(45,118)	(30,321)
Fixed Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	(16,242)	(6,780)
Held-to-maturity:
Unamortized Premium/(Discount)	(16,242)	(6,780)
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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