Business Acquisition - Schedule of Purchase Consideration and Allocation of Assets and Liabilities Assumed (Detail) - USD ($) $ in Thousands		12 Months Ended
	Mar. 21, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Purchase Consideration
Acquisition of a business, net of cash acquired		$ 110,319	$ 6,000
Assets acquired
Goodwill		$ 46,454	$ 8,028	$ 2,350
ZAC Business Acquisition [Member]
Purchase Consideration
Acquisition of a business, net of cash acquired	$ 110,319
Assets acquired
Investments	76,543
Premiums and agent's receivable	1,403
Other assets	526
Prepaid reinsurance premiums	4,792
Intangible assets value of business acquired	31,845
Total assets acquired	115,109
Total liabilities assumed	(43,216)
Net assets acquired	71,893
Goodwill	38,426
Total purchase price	110,319
ZAC Business Acquisition [Member] | Customer Relationships And Trade Name [Member]
Assets acquired
Intangible assets value of business acquired	7,600
ZAC Business Acquisition [Member] | Primarily Value Of Business Acquired [Member]
Assets acquired
Intangible assets value of business acquired	$ 24,245