Investment Securities - Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2022 USD ($) security	Dec. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	$ 3,200,026	$ 1,989,439
Unrealized loss position for less than 12 months, Unrealized Loss	(142,631)	(9,419)
Unrealized loss position for more than 12 months, Fair Value	769,973	107,592
Unrealized loss position for more than 12 months, Unrealized Loss	$ (48,827)	$ (1,055)
Investment securities:
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Number of securities in loss position | security	182	69
Unrealized loss position for more than 12 months, Number of securities in loss position | security	35	24
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	19,243	19,254
Unrealized loss position for more than 12 months, Unrealized Loss	(419)	(394)
Floating Interest Rate | Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	1,900,726	459,195
Unrealized loss position for less than 12 months, Unrealized Loss	(24,848)	(619)
Unrealized loss position for more than 12 months, Fair Value	27,935	37,307
Unrealized loss position for more than 12 months, Unrealized Loss	(687)	(477)
Fixed Interest Rate | Guaranteed mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Unrealized loss position for less than 12 months, Fair Value	704,056	406,805
Unrealized loss position for less than 12 months, Unrealized Loss	(107,687)	(5,730)
Unrealized loss position for more than 12 months, Fair Value	137,635	0
Unrealized loss position for more than 12 months, Unrealized Loss	(31,144)	0
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	595,244	1,123,439
Unrealized loss position for less than 12 months, Unrealized Loss	(10,096)	(3,070)
Unrealized loss position for more than 12 months, Fair Value	585,160	51,031
Unrealized loss position for more than 12 months, Unrealized Loss	$ (16,577)	$ (184)