Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2023 USD ($) security	Dec. 31, 2022 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	$ 1,705,315	$ 2,819,885
Unrealized loss position for less than 12 months, Unrealized Loss	(33,164)	(96,252)
Unrealized loss position for more than 12 months, Fair Value	2,443,281	1,254,553
Unrealized loss position for more than 12 months, Unrealized Loss	$ (147,983)	$ (98,280)
Investment Securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Number of securities in loss position | security	83	174
Unrealized loss position for more than 12 months, Number of securities in loss position | security	151	51
Floating Interest Rate | Floating rate auction-rate certificates backed by Government guaranteed student loans
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	$ 0	$ 0
Unrealized loss position for less than 12 months, Unrealized Loss	0	0
Unrealized loss position for more than 12 months, Fair Value	19,032	19,027
Unrealized loss position for more than 12 months, Unrealized Loss	(640)	(640)
Floating Interest Rate | Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	686,887	1,884,146
Unrealized loss position for less than 12 months, Unrealized Loss	(5,392)	(36,976)
Unrealized loss position for more than 12 months, Fair Value	1,307,943	193,964
Unrealized loss position for more than 12 months, Unrealized Loss	(27,078)	(5,934)
Fixed Interest Rate | Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	638,700	621,215
Unrealized loss position for less than 12 months, Unrealized Loss	(24,472)	(56,434)
Unrealized loss position for more than 12 months, Fair Value	650,838	336,782
Unrealized loss position for more than 12 months, Unrealized Loss	(111,943)	(74,403)
Fixed Interest Rate | Fixed rate U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	379,728	314,524
Unrealized loss position for less than 12 months, Unrealized Loss	(3,300)	(2,842)
Unrealized loss position for more than 12 months, Fair Value	465,468	704,780
Unrealized loss position for more than 12 months, Unrealized Loss	$ (8,322)	$ (17,303)