Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Sep. 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	$ 1,774,155	$ 2,819,885
Unrealized loss position for less than 12 months, Unrealized Loss	(38,277)	(96,252)
Unrealized loss position for more than 12 months, Fair Value	2,599,418	1,254,553
Unrealized loss position for more than 12 months, Unrealized Loss	$ (204,846)	$ (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	85	174
Unrealized loss position for more than 12 months, Number of securities in loss position | security	170	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	19,033	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	583,799	1,884,146
Unrealized loss position for less than 12 months, Unrealized Loss	(4,076)	(36,976)
Unrealized loss position for more than 12 months, Fair Value	1,431,925	193,964
Unrealized loss position for more than 12 months, Unrealized Loss	(30,841)	(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	535,288	621,215
Unrealized loss position for less than 12 months, Unrealized Loss	(29,649)	(56,434)
Unrealized loss position for more than 12 months, Fair Value	851,082	336,782
Unrealized loss position for more than 12 months, Unrealized Loss	(168,021)	(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	655,068	314,524
Unrealized loss position for less than 12 months, Unrealized Loss	(4,552)	(2,842)
Unrealized loss position for more than 12 months, Fair Value	297,378	704,780
Unrealized loss position for more than 12 months, Unrealized Loss	$ (5,344)	$ (17,303)