Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Sep. 30, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	$ 3,659,613
Available-for-sale, less than 12 months, gross unrealized losses	(51,166)
Available-for-sale, 12 months or more, estimated fair value	181,287
Available-for-sale, 12 months or more, gross unrealized losses	(5,955)
Available-for-sale, total, estimated fair value	3,840,900
Available-for-sale, total, gross unrealized losses	(57,121)
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	190,275
Available-for-sale, less than 12 months, gross unrealized losses	(3,759)
Available-for-sale, 12 months or more, estimated fair value	111,662
Available-for-sale, 12 months or more, gross unrealized losses	(3,946)
Available-for-sale, total, estimated fair value	301,937
Available-for-sale, total, gross unrealized losses	(7,705)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	2,342,244
Available-for-sale, less than 12 months, gross unrealized losses	(27,380)
Available-for-sale, 12 months or more, estimated fair value	46,253
Available-for-sale, 12 months or more, gross unrealized losses	(722)
Available-for-sale, total, estimated fair value	2,388,497
Available-for-sale, total, gross unrealized losses	(28,102)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	969,120
Available-for-sale, less than 12 months, gross unrealized losses	(16,056)
Available-for-sale, 12 months or more, estimated fair value	23,372
Available-for-sale, 12 months or more, gross unrealized losses	(1,287)
Available-for-sale, total, estimated fair value	992,492
Available-for-sale, total, gross unrealized losses	(17,343)
Other securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	157,974
Available-for-sale, less than 12 months, gross unrealized losses	(3,971)
Available-for-sale, 12 months or more, estimated fair value	0
Available-for-sale, 12 months or more, gross unrealized losses	0
Available-for-sale, total, estimated fair value	157,974
Available-for-sale, total, gross unrealized losses	$ (3,971)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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