Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Held-to-maturity:
Allowance for losses	$ (216)
Investment securities
Available-for-sale:
Amount Outstanding	3,513,235	$ 2,960,048
Unamortized Premium/(Discount)	9,439	1,382
Amortized cost of available-for-sale investment securities	3,522,674	2,961,430
Allowance for losses	(37)
Unrealized Gains	13,309	3,959
Unrealized Losses	(3,756)	(5,546)
Available-for-sale, at fair value	3,532,190	2,959,843
Held-to-maturity:
Unamortized Premium/(Discount)	9,439	1,382
Total investment securities
Amount Outstanding	3,558,267	3,005,080
Unamortized Premium/(Discount)	9,439	1,382
Amortized Cost	3,567,706	3,006,462
Allowance for losses	(37)
Unrealized Gains	14,462	4,912
Unrealized Losses	(3,756)	(5,546)
Fair Value	3,578,375	3,005,828
Accrued interest excluded	5,700
Available-for-Sale Securities \| Investment securities
Available-for-sale:
Unamortized Premium/(Discount)	9,439	1,382
Held-to-maturity:
Unamortized Premium/(Discount)	9,439	1,382
Total investment securities
Unamortized Premium/(Discount)	9,439	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	(37)
Unrealized Gains	0	0
Unrealized Losses	(591)	(788)
Available-for-sale, at fair value	19,072	18,912
Floating Interest Rate \| Floating rate asset-backed securities
Available-for-sale:
Amount Outstanding	9,205	11,092
Amortized cost of available-for-sale investment securities	9,205	11,092
Allowance for losses	0
Unrealized Gains	0	0
Unrealized Losses	(4)	(7)
Available-for-sale, at fair value	9,201	11,085
Floating Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,279,737	1,633,731
Amortized cost of available-for-sale investment securities	2,279,770	1,634,905
Allowance for losses	0
Unrealized Gains	9,415	2,414
Unrealized Losses	(3,152)	(4,736)
Available-for-sale, at fair value	2,286,033	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)	33	1,174
Held-to-maturity:
Unamortized Premium/(Discount)	33	1,174
Total investment securities
Unamortized Premium/(Discount)	33	1,174
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	285	315
Amortized cost of available-for-sale investment securities	285	315
Allowance for losses	0
Unrealized Gains	33	25
Unrealized Losses	0	0
Available-for-sale, at fair value	318	340
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Allowance for losses	0
Unrealized Gains	1,153	953
Unrealized Losses	0	0
Fair Value	$ 46,185	$ 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	$ 1,204,308	$ 1,295,210
Amortized cost of available-for-sale investment securities	1,213,714	1,295,418
Allowance for losses	0
Unrealized Gains	3,861	1,520
Unrealized Losses	(9)	(15)
Available-for-sale, at fair value	1,217,566	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)	9,406	208
Held-to-maturity:
Unamortized Premium/(Discount)	9,406	208
Total investment securities
Unamortized Premium/(Discount)	9,406	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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