Reinsurance Written Premium (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,613,825	$ 1,489,422	$ 1,403,865
Ceded to Other Companies									441,876	441,935	382,687
Assumed from Other Companies									299,136	223,843	165,046
Net written premiums	$ 346,119	$ 352,412	$ 379,292	$ 393,262	$ 304,869	$ 296,016	$ 333,282	$ 337,163	$ 1,471,085	$ 1,271,330	$ 1,186,224
Percentage of Amount Assumed to Net									20.30%	17.60%	13.90%
Accident & Health
Supplementary Insurance Information, by Segment [Line Items]
Assumed from Other Companies									$ 139,692	$ 80,605	$ 53,814
Net written premiums									$ 139,692	$ 80,605	$ 53,814
Percentage of Amount Assumed to Net									100.00%	100.00%	100.00%
Property Casualty
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,613,825	$ 1,489,422	$ 1,403,865
Ceded to Other Companies									441,876	441,935	382,687
Assumed from Other Companies									159,444	143,238	111,232
Net written premiums									$ 1,331,393	$ 1,190,725	$ 1,132,410
Percentage of Amount Assumed to Net									12.00%	12.00%	9.80%