Investments - Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Investment Income, Net [Abstract]
Interest income	$ 109,000	$ 99,000	$ 209,300	$ 199,000
Dividend income	12,800	29,900	25,500	46,600
Investment expenses	(7,400)	(6,900)	(14,700)	(12,900)
Partnerships and other investment results	(8,900)	4,900	(1,200)	47,700
Total	$ 105,452	$ 126,931	$ 218,928	$ 280,415