Subsequent Events - Additional Information (Detail)	3 Months Ended			9 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Nov. 02, 2016 Claim
Subsequent Event [Line Items]
Incurred (recovered) related to current year		$ 73,701,000	$ 54,014,000	$ 199,886,000	$ 127,211,000
Hurricane Mathew [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Number of claims reported | Claim						5,657
Hurricane Mathew [Member] | Scenario Forecast [Member] | Minimum [Member]
Subsequent Event [Line Items]
Incurred (recovered) related to current year	$ 14,000,000
Hurricane Mathew [Member] | Scenario Forecast [Member] | Maximum [Member]
Subsequent Event [Line Items]
Incurred (recovered) related to current year	$ 18,000,000