Investments - Summary of Unaudited Financial Information and Unaudited Financial Position of Limited Partnerships (Detail) - USD ($) $ in Thousands		3 Months Ended												12 Months Ended
		Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Equity Method Investments [Line Items]
Total income		$ 162,670	$ 131,644	$ 127,327	$ 129,029	$ 119,943	$ 126,654	$ 125,926	$ 127,040	$ 112,320	$ 99,217	$ 101,504	$ 94,874	$ 550,670	$ 499,563	$ 407,915
Net income		40,913	$ 15,669	$ 14,882	$ 17,793	2,651	$ (51,503)	$ (8,542)	$ 2,791	$ 1,435	$ (4,868)	$ 3,830	$ 6,845	89,257	(54,603)	7,242
Balance sheet:
Total assets		1,811,316				1,803,328								1,811,316	1,803,328
Total liabilities		1,387,991				1,548,521								1,387,991	1,548,521
Limited Partnership [Member]
Schedule Of Equity Method Investments [Line Items]
Total income	[1]													1,606	1,252,264	705,610
Total expenses														(71,256)	(139,174)	(131,463)
Net income														(69,650)	1,113,090	$ 574,147
Balance sheet:
Total assets		4,072,501				5,119,695								4,072,501	5,119,695
Total liabilities		$ 220,525				$ 430,354								$ 220,525	$ 430,354

[1] Includes net change in unrealized gains or losses on investments.