Amount of written premiums assumed from other entities.

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Amount of written premiums ceded to other entities.

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Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written.

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Percentage of assumed premiums earned by the entity to net premiums earned.

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Reinsurance Written Premium (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,489,422	$ 1,403,865	$ 1,277,728
Ceded to Other Companies									441,935	382,687	409,642
Assumed from Other Companies									223,843	165,046	175,774
Net written premiums	$ 304,869	$ 296,016	$ 333,282	$ 337,163	$ 282,868	$ 277,001	$ 306,535	$ 319,820	$ 1,271,330	$ 1,186,224	$ 1,043,860
Percentage of Amount Assumed to Net									18.00%	14.00%	17.00%
Accident & Health
Supplementary Insurance Information, by Segment [Line Items]
Assumed from Other Companies									$ 80,605	$ 53,814	$ 87,088
Net written premiums									$ 80,605	$ 53,814	$ 87,088
Percentage of Amount Assumed to Net									100.00%	100.00%	100.00%
Property Casualty
Supplementary Insurance Information, by Segment [Line Items]
Direct Amount									$ 1,489,422	$ 1,403,865	$ 1,277,728
Ceded to Other Companies									441,935	382,687	409,642
Assumed from Other Companies									143,238	111,232	88,686
Net written premiums									$ 1,190,725	$ 1,132,410	$ 956,772
Percentage of Amount Assumed to Net									12.00%	10.00%	9.00%