Property and Casualty Premiums Written and Earned (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effects of Reinsurance [Line Items]
Net premiums written	$ 5,091,800	$ 4,489,200	$ 4,497,500
Net premiums earned	4,975,777	4,230,286	4,410,647
Property and Casualty Insurance
Effects of Reinsurance [Line Items]
Gross premiums written - direct	1,490,300	1,505,600	1,529,400
Gross premiums written - assumed	4,276,800	3,616,600	3,567,200
Ceded premiums written	(675,300)	(633,000)	(599,100)
Net premiums written	5,091,800	4,489,200	4,497,500
Gross premiums earned - direct	1,871,100	1,515,900	1,517,000
Gross premiums earned - assumed	3,833,900	3,403,300	3,540,500
Ceded premiums earned	(729,200)	(688,900)	(646,900)
Net premiums earned	$ 4,975,800	$ 4,230,300	$ 4,410,600