Reinsurance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 1,932,000	$ 1,871,100	$ 1,515,900
Ceded to Other Companies	767,300	729,200	688,900
Assumed from Other Companies	3,790,300	3,833,900	3,403,300
Net Amount	4,954,990	4,975,777	4,230,286
Property and Casualty Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	1,932,000	1,871,100	1,515,900
Ceded to Other Companies	767,300	729,200	688,900
Assumed from Other Companies	3,790,300	3,833,900	3,403,300
Net Amount	$ 4,955,000	$ 4,975,800	$ 4,230,300
Percentage of Amount Assumed to Net	76.50%	77.10%	80.50%