Investment Securities - Schedule of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Investment securities:
Available-for-sale:
Amount Outstanding	$ 4,839,001	$ 4,806,727
Unamortized Premium/(Discount)	0	0
Amortized cost	4,788,254	4,769,426
Allowance for losses	(29)	(33)
Unrealized Gains	9,989	4,703
Unrealized Losses	(150,750)	(194,532)
Fair Value	4,647,464	4,579,564
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	611	2,433
Unrealized Losses	0	0
Fair Value	45,643	47,465
Available-for-sale, accrued interest excluded	12,400	10,600
Available-for-Sale Securities \| Investment securities:
Available-for-sale:
Unamortized Premium/(Discount)	(50,747)	(37,301)
Held-to-maturity:
Unamortized Premium/(Discount)	(50,747)	(37,301)
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	(29)	(33)
Unrealized Gains	0	0
Unrealized Losses	(640)	(640)
Fair Value	19,031	19,027
Floating Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	2,472,591	2,433,696
Amortized cost	2,471,502	2,433,496
Allowance for losses	0	0
Unrealized Gains	3,525	1,954
Unrealized Losses	(24,442)	(42,910)
Fair Value	2,450,585	2,392,540
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
Floating Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	(1,089)	(200)
Held-to-maturity:
Unamortized Premium/(Discount)	(1,089)	(200)
Fixed Interest Rate \| Guaranteed mortgage-backed securities
Available-for-sale:
Amount Outstanding	1,404,950	1,207,416
Amortized cost	1,363,000	1,177,095
Allowance for losses	0	0
Unrealized Gains	4,939	2,128
Unrealized Losses	(113,091)	(130,837)
Fair Value	1,254,848	1,048,386
Held-to-maturity:
Amount Outstanding	45,032	45,032
Amortized Cost	45,032	45,032
Allowance for losses	0	0
Unrealized Gains	611	2,433
Unrealized Losses	0	0
Fair Value	$ 45,643	$ 47,465
Held-to-maturity investment securities weighted average yield	5.80%	4.50%
Fixed Interest Rate \| Fixed rate U.S. Treasuries
Available-for-sale:
Amount Outstanding	$ 941,760	$ 1,145,915
Amortized cost	934,052	1,139,135
Allowance for losses	0	0
Unrealized Gains	1,525	621
Unrealized Losses	(12,577)	(20,145)
Fair Value	923,000	1,119,611
Fixed Interest Rate \| Available-for-Sale Securities \| Guaranteed mortgage-backed securities
Available-for-sale:
Unamortized Premium/(Discount)	(41,950)	(30,321)
Held-to-maturity:
Unamortized Premium/(Discount)	(41,950)	(30,321)
Fixed Interest Rate \| Available-for-Sale Securities \| Fixed rate U.S. Treasuries
Available-for-sale:
Unamortized Premium/(Discount)	(7,708)	(6,780)
Held-to-maturity:
Unamortized Premium/(Discount)	(7,708)	(6,780)
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

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