Securities - Narrative (Details)	1 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	May 31, 2023 USD ($)	Sep. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) contract
Debt Securities, Available-for-sale [Line Items]
Securities		$ 1,314,367,000			$ 1,314,367,000		$ 1,366,680,000
Debt securities available for sale, at fair value		1,033,797,000			1,033,797,000		1,057,621,000
Allowance for credit losses		0			0		0
Total unrealized loss		1,008,906,000			1,008,906,000		1,002,437,000
Equity securities, unrealized loss		100,000			100,000
Pre-tax loss		0		$ 0	10,760,000	$ 0
Equity securities, original costs		2,500,000			2,500,000
Equity securities with readily determinable fair value not held for trading		2,438,000			2,438,000		11,383,000
Unrealized gains				$ 1,500,000		$ 400,000
Foreign Corporate Debt Securities
Debt Securities, Available-for-sale [Line Items]
Debt securities available for sale, at fair value		0			0		0
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Debt securities available for sale, at fair value		252,578,000			252,578,000		280,700,000
Allowance for credit losses		0			0		0
Total unrealized loss		252,578,000			252,578,000		275,706,000
Proceeds from sale of debt securities, available-for-sale	$ 800,000
Pre-tax loss	$ 1,200,000
Investment-Grade
Debt Securities, Available-for-sale [Line Items]
Debt securities available for sale, at fair value		182,200,000			182,200,000		206,300,000
Unrealized loss					22,900,000		16,600,000
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Debt securities available for sale, at fair value			$ 11,200,000
Realized loss			$ 200,000
Pledged as Collateral
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral		159,500,000			159,500,000		314,500,000
Foreign Corporate Debt Securities
Debt Securities, Available-for-sale [Line Items]
Securities		9,800,000			9,800,000		9,700,000
US Government
Debt Securities, Available-for-sale [Line Items]
Allowance for credit losses		0			0		0
Investment-Grade
Debt Securities, Available-for-sale [Line Items]
Securities		229,700,000			229,700,000		258,800,000
Allowance for credit losses		0			0		0
Total unrealized loss		28,700,000			28,700,000		24,100,000
Non-Investment-Grade
Debt Securities, Available-for-sale [Line Items]
Securities		22,900,000			22,900,000		16,900,000
Total unrealized loss		$ 4,100,000			4,100,000		2,100,000
Signature Bond
Debt Securities, Available-for-sale [Line Items]
Total unrealized loss							$ 9,100,000
Number of instruments held-for-sale | contract							1
Equity securities, unrealized loss							$ 900,000
Realized loss					$ 9,500,000
Mutual funds
Debt Securities, Available-for-sale [Line Items]
Equity securities, original costs							$ 12,700,000