Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Investment Income Net [Abstract]
Interest income	$ 99,000	$ 109,200	$ 199,000	$ 228,500
Dividend income	29,900	5,200	46,600	15,200
Investment expenses	(6,900)	(7,200)	(12,900)	(14,400)
Partnerships and other investment results	4,900	11,500	47,700	1,400
Net investment income	$ 126,931	$ 118,691	$ 280,415	$ 230,673