Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investment Income Net [Abstract]
Interest income	$ 109,100	$ 130,100	$ 337,600	$ 376,600
Dividend income	4,900	9,700	20,200	28,900
Investment expenses	(7,600)	(7,400)	(22,100)	(22,100)
Partnerships and other investment results	22,900	15,400	24,200	30,200
Net investment income	$ 129,265	$ 147,829	$ 359,938	$ 413,623