Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment securities:
Available-for-sale:
Amount Outstanding	$ 3,786,758	$ 3,826,582
Unamortized Premium/(Discount)	0	0
Amortized Cost	3,800,537	3,843,666
Allowance for losses	(61)	(36)
Unrealized Gains	12,635	13,642
Unrealized Losses	(3,479)	(3,580)
Fair Value	3,809,632	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	1,552	1,201
Unrealized Losses	0	0
Fair Value	46,584	46,233
Accrued interest excluded	6,400	9,000
Available-for-Sale Securities \| Investment securities:
Available-for-sale:
Unamortized Premium/(Discount)	13,779	17,084
Held-to-maturity:
Unamortized Premium/(Discount)	13,779	17,084
Total held-to-maturity
Unamortized Premium/(Discount)	13,779	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	(61)	(36)
Unrealized Gains	0	0
Unrealized Losses	(493)	(493)
Fair Value	19,146	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	2,378,471	2,350,963
Amortized Cost	2,378,436	2,350,919
Allowance for losses	0	0
Unrealized Gains	12,059	12,150
Unrealized Losses	(2,937)	(3,043)
Fair Value	2,387,558	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)	(35)	(44)
Held-to-maturity:
Unamortized Premium/(Discount)	(35)	(44)
Total held-to-maturity
Unamortized Premium/(Discount)	(35)	(44)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	179	279
Amortized Cost	179	279
Allowance for losses	0	0
Unrealized Gains	22	34
Unrealized Losses	0	0
Fair Value	201	313
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Allowance for losses	0	0
Unrealized Gains	1,552	1,201
Unrealized Losses	0	0
Fair Value	$ 46,584	$ 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	$ 1,388,408	$ 1,449,408
Amortized Cost	1,402,222	1,466,536
Allowance for losses	0	0
Unrealized Gains	554	1,458
Unrealized Losses	(49)	(43)
Fair Value	1,402,727	1,467,951
Fixed Interest Rate \| Available-for-Sale 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
Fixed Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	13,814	17,128
Held-to-maturity:
Unamortized Premium/(Discount)	13,814	17,128
Total held-to-maturity
Unamortized Premium/(Discount)	13,814	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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