Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Investment securities:
Available-for-sale:
Amount Outstanding	$ 4,244,839	$ 3,819,376
Unamortized Premium/(Discount)	0	0
Amortized cost	4,258,227	3,834,714
Allowance for losses	(49)	(52)
Unrealized Gains	7,324	12,203
Unrealized Losses	(70,188)	(10,474)
Fair Value	4,195,314	3,836,391
Held-to-maturity:
Unamortized Premium/(Discount)	0	0
Total held-to-maturity
Amount Outstanding	44,970	44,970
Unamortized Premium/(Discount)	0	0
Amortized Cost	44,970	44,970
Allowance for losses	0	0
Unrealized Gains	0	1,612
Unrealized Losses	(371)	0
Fair Value	44,599	46,582
Available-for-sale, accrued interest excluded	3,900	4,300
Available-for-Sale Securities \| Investment securities:
Available-for-sale:
Unamortized Premium/(Discount)	13,388	15,338
Held-to-maturity:
Unamortized Premium/(Discount)	13,388	15,338
Total held-to-maturity
Unamortized Premium/(Discount)	13,388	15,338
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	(49)	(52)
Unrealized Gains	0	0
Unrealized Losses	(690)	(394)
Fair Value	18,961	19,254
Floating Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,261,234	2,168,016
Amortized cost	2,260,873	2,168,106
Allowance for losses	0	0
Unrealized Gains	7,314	11,821
Unrealized Losses	(3,030)	(1,096)
Fair Value	2,265,157	2,178,831
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
Total 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)	(361)	90
Held-to-maturity:
Unamortized Premium/(Discount)	(361)	90
Total held-to-maturity
Unamortized Premium/(Discount)	(361)	90
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	554,118	451,660
Amortized cost	569,299	464,185
Allowance for losses	0	0
Unrealized Gains	9	382
Unrealized Losses	(48,688)	(5,730)
Fair Value	520,620	458,837
Held-to-maturity:
Amount Outstanding	44,970	44,970
Amortized Cost	44,970	44,970
Allowance for losses	0	0
Unrealized Gains	0	1,612
Unrealized Losses	(371)	0
Fair Value	$ 44,599	$ 46,582
Total held-to-maturity
Held-to-maturity investment securities weighted average yield	1.50%	1.50%
Fixed Interest Rate \| Fixed rate U.S. Treasuries
Available-for-sale:
Amount Outstanding	$ 1,409,787	$ 1,180,000
Amortized cost	1,408,355	1,182,723
Allowance for losses	0	0
Unrealized Gains	1	0
Unrealized Losses	(17,780)	(3,254)
Fair Value	1,390,576	1,179,469
Fixed Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	15,181	12,525
Held-to-maturity:
Unamortized Premium/(Discount)	15,181	12,525
Total held-to-maturity
Unamortized Premium/(Discount)	15,181	12,525
Fixed Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	(1,432)	2,723
Held-to-maturity:
Unamortized Premium/(Discount)	(1,432)	2,723
Total held-to-maturity
Unamortized Premium/(Discount)	(1,432)	2,723
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
Total held-to-maturity
Unamortized Premium/(Discount)	$ 0	$ 0

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