Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Investment Income Net [Abstract]
Interest income	$ 109,200	$ 127,200	$ 228,500	$ 246,500
Dividend income	5,200	9,400	15,200	19,200
Investment expenses	(7,200)	(7,200)	(14,400)	(14,700)
Partnerships and other investment results	11,500	13,300	1,400	14,800
Net investment income	$ 118,691	$ 142,745	$ 230,673	$ 265,794