Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Investment Income Net [Abstract]
Interest income	$ 99,900	$ 119,300
Dividend income	16,700	10,100
Investment expenses	(6,000)	(7,100)
Partnerships and other investment results	42,900	(10,300)
Net investment income	$ 153,484	$ 111,982