Fair Value Measurements and Derivative Instruments (Unrealized Holding Losses) (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Greater than 12 continuous months, Fair Value	$ 190,699	$ 199,879
Greater than 12 continuous months, Gross unrealized losses	45,068	43,605
Less than 12 continuous months, Fair Value	1,859	3,963
Less than 12 continuous months, Gross unrealized losses	226	419
Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Greater than 12 continuous months, Fair Value	23,763	28,030
Greater than 12 continuous months, Gross unrealized losses	6,873	7,018
Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Greater than 12 continuous months, Fair Value	51,879	51,948
Greater than 12 continuous months, Gross unrealized losses	10,961	10,466
Less than 12 continuous months, Fair Value	284	2,382
Less than 12 continuous months, Gross unrealized losses	2	212
Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Less than 12 continuous months, Fair Value	1,575	1,581
Less than 12 continuous months, Gross unrealized losses	224	207
Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Greater than 12 continuous months, Fair Value	115,057	119,901
Greater than 12 continuous months, Gross unrealized losses	$ 27,234	$ 26,121

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.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
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Amount of accumulated unrealized loss on 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 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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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 less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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