Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment securities:
Available-for-sale:
Amount Outstanding	$ 3,826,582	$ 2,960,048
Unamortized Premium/(Discount)	17,084	1,382
Amortized cost of available-for-sale investment securities	3,843,666	2,961,430
Allowance for losses	(36)
Unrealized Gains	13,642	3,959
Unrealized Losses	(3,580)	(5,546)
Available-for-sale, at fair value	3,853,692	2,959,843
Held-to-maturity:
Unamortized Premium/(Discount)	17,084	1,382
Total investment securities
Amount Outstanding	3,871,614	3,005,080
Unamortized Premium/(Discount)	17,084	1,382
Amortized Cost	3,888,698	3,006,462
Allowance for losses	(36)
Unrealized Gains	14,843	4,912
Unrealized Losses	(3,580)	(5,546)
Fair Value	3,899,925	3,005,828
Accrued interest excluded	9,000
Available-for-Sale Securities \| Investment securities:
Available-for-sale:
Unamortized Premium/(Discount)	17,084	1,382
Held-to-maturity:
Unamortized Premium/(Discount)	17,084	1,382
Total investment securities
Unamortized Premium/(Discount)	17,084	1,382
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 of available-for-sale investment securities	19,700	19,700
Allowance for losses	(36)
Unrealized Gains	0	0
Unrealized Losses	(493)	(788)
Available-for-sale, at fair value	19,171	18,912
Floating Interest Rate \| Floating rate asset-backed securities
Available-for-sale:
Amount Outstanding	6,232	11,092
Amortized cost of available-for-sale investment securities	6,232	11,092
Allowance for losses	0
Unrealized Gains	0	0
Unrealized Losses	(1)	(7)
Available-for-sale, at fair value	6,231	11,085
Floating Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,350,963	1,633,731
Amortized cost of available-for-sale investment securities	2,350,919	1,634,905
Allowance for losses	0
Unrealized Gains	12,150	2,414
Unrealized Losses	(3,043)	(4,736)
Available-for-sale, at fair value	2,360,026	1,632,583
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 investment securities
Unamortized Premium/(Discount)	0	0
Floating Interest Rate \| Available-for-Sale Securities \| Floating rate asset-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	0	0
Held-to-maturity:
Unamortized Premium/(Discount)	0	0
Total investment securities
Unamortized Premium/(Discount)	0	0
Floating Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	(44)	1,174
Held-to-maturity:
Unamortized Premium/(Discount)	(44)	1,174
Total investment securities
Unamortized Premium/(Discount)	(44)	1,174
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	279	315
Amortized cost of available-for-sale investment securities	279	315
Allowance for losses	0
Unrealized Gains	34	25
Unrealized Losses	0	0
Available-for-sale, at fair value	313	340
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Allowance for losses	0
Unrealized Gains	1,201	953
Unrealized Losses	0	0
Fair Value	$ 46,233	$ 45,985
Total investment securities
Held-to-maturity investment securities weighted average yield	1.50%	3.30%
Fixed Interest Rate \| Fixed rate U.S. Treasuries
Available-for-sale:
Amount Outstanding	$ 1,449,408	$ 1,295,210
Amortized cost of available-for-sale investment securities	1,466,536	1,295,418
Allowance for losses	0
Unrealized Gains	1,458	1,520
Unrealized Losses	(43)	(15)
Available-for-sale, at fair value	1,467,951	1,296,923
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 investment securities
Unamortized Premium/(Discount)	0	0
Fixed Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	17,128	208
Held-to-maturity:
Unamortized Premium/(Discount)	17,128	208
Total investment securities
Unamortized Premium/(Discount)	17,128	208
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 investment securities
Unamortized Premium/(Discount)	$ 0	$ 0

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