Investment Securities - Securities With Unrealized Losses (Details) $ in Thousands	Mar. 31, 2020 USD ($)
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	$ 507,520
Available-for-sale, less than 12 months, gross unrealized losses	(2,518)
Available-for-sale, 12 months or more, estimated fair value	133,004
Available-for-sale, 12 months or more, gross unrealized losses	(2,686)
Available-for-sale, total, estimated fair value	640,524
Available-for-sale, total, gross unrealized losses	(5,204)
U.S. Treasury
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	424,989
Available-for-sale, less than 12 months, gross unrealized losses	(43)
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	424,989
Available-for-sale, total, gross unrealized losses	(43)
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	0
Available-for-sale, less than 12 months, gross unrealized losses	0
Available-for-sale, 12 months or more, estimated fair value	58,056
Available-for-sale, 12 months or more, gross unrealized losses	(891)
Available-for-sale, total, estimated fair value	58,056
Available-for-sale, total, gross unrealized losses	(891)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	57,718
Available-for-sale, less than 12 months, gross unrealized losses	(1,318)
Available-for-sale, 12 months or more, estimated fair value	74,523
Available-for-sale, 12 months or more, gross unrealized losses	(1,791)
Available-for-sale, total, estimated fair value	132,241
Available-for-sale, total, gross unrealized losses	(3,109)
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, less than 12 months, estimated fair value	24,813
Available-for-sale, less than 12 months, gross unrealized losses	(1,157)
Available-for-sale, 12 months or more, estimated fair value	425
Available-for-sale, 12 months or more, gross unrealized losses	(4)
Available-for-sale, total, estimated fair value	25,238
Available-for-sale, total, gross unrealized losses	$ (1,161)

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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