Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investment securities:
Available-for-sale:
Amount Outstanding	$ 3,673,743	$ 3,826,582
Unamortized Premium/(Discount)	0	0
Amortized cost	3,684,066	3,843,666
Allowance for losses	(52)	(36)
Unrealized Gains	15,010	13,642
Unrealized Losses	(2,820)	(3,580)
Fair Value	3,696,204	3,853,692
Held-to-maturity:
Unamortized Premium/(Discount)	0	0
Total 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	996	1,201
Unrealized Losses	0	0
Fair Value	46,028	46,233
Available-for-sale, accrued interest excluded	4,700	9,000
Available-for-Sale Securities \| Investment securities:
Available-for-sale:
Unamortized Premium/(Discount)	10,323	17,084
Held-to-maturity:
Unamortized Premium/(Discount)	10,323	17,084
Total held-to-maturity
Unamortized Premium/(Discount)	10,323	17,084
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	(52)	(36)
Unrealized Gains	0	0
Unrealized Losses	(148)	(493)
Fair Value	19,500	19,171
Floating Interest Rate \| Floating rate asset-backed securities
Available-for-sale:
Amount Outstanding		6,232
Amortized cost		6,232
Allowance for losses		0
Unrealized Gains		0
Unrealized Losses		(1)
Fair Value		6,231
Floating Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,386,373	2,350,963
Amortized cost	2,390,500	2,350,919
Allowance for losses	0	0
Unrealized Gains	14,827	12,150
Unrealized Losses	(2,253)	(3,043)
Fair Value	2,403,074	2,360,026
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 \| Floating rate asset-backed securities
Available-for-sale:
Unamortized Premium/(Discount)		0
Held-to-maturity:
Unamortized Premium/(Discount)		0
Total held-to-maturity
Unamortized Premium/(Discount)		0
Floating Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	4,127	(44)
Held-to-maturity:
Unamortized Premium/(Discount)	4,127	(44)
Total held-to-maturity
Unamortized Premium/(Discount)	4,127	(44)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	28,170	279
Amortized cost	28,961	279
Allowance for losses	0	0
Unrealized Gains	21	34
Unrealized Losses	(85)	0
Fair Value	28,897	313
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Allowance for losses	0	0
Unrealized Gains	996	1,201
Unrealized Losses	0	0
Fair Value	$ 46,028	$ 46,233
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	$ 1,239,500	$ 1,449,408
Amortized cost	1,244,905	1,466,536
Allowance for losses	0	0
Unrealized Gains	162	1,458
Unrealized Losses	(334)	(43)
Fair Value	1,244,733	1,467,951
Fixed Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	791	0
Held-to-maturity:
Unamortized Premium/(Discount)	791	0
Total held-to-maturity
Unamortized Premium/(Discount)	791	0
Fixed Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	5,405	17,128
Held-to-maturity:
Unamortized Premium/(Discount)	5,405	17,128
Total held-to-maturity
Unamortized Premium/(Discount)	5,405	17,128
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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