Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2022

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Vale on a Recurring Basis

All assets measured at fair value are categorized in the table below based upon the lowest level of significant input to the valuations. All fair value measurements at the reporting date were on a recurring basis.

December 31, 2022 (In thousands)	Quoted Prices in Active  Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Recurring fair value measurements
Financial Assets:
U.S. Treasury securities	$	556,438	$	—	$	—	$	556,438
U.S. agency securities	—		49,058		—		49,058
U.S. agency mortgage-backed securities	—		783,743		—		783,743
Municipal debt securities	—		602,690		—		602,690
Non-U.S. government securities	—		62,399		—		62,399
Corporate debt securities	—		1,414,321		—		1,414,321
Residential and commercial mortgage securities	—		511,824		—		511,824
Asset-backed securities	—		624,561		—		624,561
Money market funds	136,591		—		—		136,591
Total assets at fair value (1) (2)	$	693,029	$	4,048,596	$	—	$	4,741,625

December 31, 2021 (In thousands)	Quoted Prices in Active  Markets for Identical Instruments (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)		Total
Recurring fair value measurements
Financial Assets:
U.S. Treasury securities	$	448,793	$	—	$	—	$	448,793
U.S. agency securities	—		5,504		—		5,504
U.S. agency mortgage-backed securities	—		1,008,863		—		1,008,863
Municipal debt securities	—		627,599		—		627,599
Non-U.S. government securities	—		79,743		—		79,743
Corporate debt securities	—		1,455,247		—		1,455,247
Residential and commercial mortgage securities	—		545,423		—		545,423
Asset-backed securities	—		581,703		—		581,703
Money market funds	210,012		—		—		210,012
Total assets at fair value (1)	$	658,805	$	4,304,082	$	—	$	4,962,887

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(1)Does not include the fair value of embedded derivatives, which we have accounted for separately as freestanding derivatives and included in other assets or other accrued liabilities in our consolidated balance sheet. See Note 5 for more information.

(2)Does not include certain other invested assets that are measured at fair value using the net asset value per share (or its equivalent) as a practical expedient, as applicable accounting standards do not provide for classification within the fair value hierarchy.