Summary of Significant Accounting Policies - Additional Information (Detail)		9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Entity	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Significant Accounting Policies [Line Items]
Limited liability partnerships, percentage of ownership interest			3.00%
Investments in other invested assets			$ 66,121,000	$ 32,592,000
Cash and cash equivalents			75,110,000	67,037,000	$ 58,823,000	$ 105,492,000
Allowance for bad debts			1,900,000	1,600,000
Impairment of goodwill			0	0
Impairments of indefinite lived intangible assets			0	0
Impairment of definite lived intangible assets			0	0
Amortization of deferred acquisition costs			114,300,000	86,200,000	57,100,000
Premium deficiency charges			300,000	200,000	400,000
Premium deficiency reserve			0	0
Net foreign currency transaction gains (losses)			(700,000)	400,000	500,000
Proceeds from sale of capital stock	$ 18,700,000	$ 18,700,000
Pretax gain on sale of capital stock	$ 6,900,000	$ 6,900,000	$ 6,857,000
Variable Interest Entity, Not Primary Beneficiary
Significant Accounting Policies [Line Items]
Number of VIEs | Entity			2
Ownership interest exceeds respective investments			3.00%
Allowance for Reinsurance Recoverable
Significant Accounting Policies [Line Items]
Allowance for uncollectible reinsurance receivables			$ 8,040,000	$ 9,675,000	$ 9,350,000	$ 9,010,000
Money Market Funds
Significant Accounting Policies [Line Items]
Cash and cash equivalents			$ 52,000,000