Net Investment Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Investment Income Net [Abstract]
Interest income	$ 119,300	$ 119,200
Dividend income	10,100	9,800
Investment expenses	(7,100)	(7,500)
Partnerships and other investment results	(10,300)	1,500
Net investment income	$ 111,982	$ 123,049