Net Investment Income (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Investment Income [Line Items]
Interest income		$ 412,100	$ 385,700	$ 376,400
Dividend income		47,400	50,300	54,000
Investment expenses		(28,100)	(26,300)	(27,200)
Other investment results		18,000	1,500	12,000
Net investment income		451,016	438,455	438,817
Ares
Net Investment Income [Line Items]
Equity results	[1]	3,300	11,500	6,600
ORX
Net Investment Income [Line Items]
Equity results				(6,300)
Pillar Holdings and Funds | Pillar Capital Holdings Limited And Managed Funds
Net Investment Income [Line Items]
Equity results	[1]	$ (1,700)	$ 15,800	$ 23,300

[1] See Note 4(h) for discussion of the Pillar Investments and the investment in Ares, each as defined therein.