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Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Mar. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	$ 2,373,301
Available-for-sale, less than 12 months, gross unrealized losses	(90,561)
Available-for-sale, 12 months or more, estimated fair value	49,536
Available-for-sale, 12 months or more, gross unrealized losses	(780)
Available-for-sale, total, estimated fair value	2,422,837
Available-for-sale, total, gross unrealized losses	(91,341)
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	402,235
Available-for-sale, less than 12 months, gross unrealized losses	(23,887)
Available-for-sale, 12 months or more, estimated fair value	49,123
Available-for-sale, 12 months or more, gross unrealized losses	(776)
Available-for-sale, total, estimated fair value	451,358
Available-for-sale, total, gross unrealized losses	(24,663)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	1,622,032
Available-for-sale, less than 12 months, gross unrealized losses	(35,949)
Available-for-sale, 12 months or more, estimated fair value	323
Available-for-sale, 12 months or more, gross unrealized losses	(3)
Available-for-sale, total, estimated fair value	1,622,355
Available-for-sale, total, gross unrealized losses	(35,952)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	299,613
Available-for-sale, less than 12 months, gross unrealized losses	(24,624)
Available-for-sale, 12 months or more, estimated fair value	90
Available-for-sale, 12 months or more, gross unrealized losses	(1)
Available-for-sale, total, estimated fair value	299,703
Available-for-sale, total, gross unrealized losses	(24,625)
Other securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	49,421
Available-for-sale, less than 12 months, gross unrealized losses	(6,101)
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	49,421
Available-for-sale, total, gross unrealized losses	$ (6,101)

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- Definition

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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