Reinsurance Balances Recoverable - Additional Information (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2016 USD ($) reinsurer	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) reinsurer
Effects of Reinsurance [Line Items]
Reinsurance balances recoverable	$ 1,345,115		$ 1,474,004
Increase (decrease) in reinsurance recoverable	$ (132,938)	$ (210,401)
Reinsurer Concentration Risk | Top Ten Reinsurers, Seven Reinsurers Rated A Minus Or Higher
Effects of Reinsurance [Line Items]
Number of external reinsurers | reinsurer	7		7
Reinsurer Concentration Risk | Top Ten Reinsurers, Three Non-Rated Reinsurers
Effects of Reinsurance [Line Items]
Reinsurance balances recoverable	$ 297,000		$ 337,600
Reinsurer Concentration Risk | Top Ten Reinsurers, Lloyd's Syndicates
Effects of Reinsurance [Line Items]
Reinsurance balances recoverable	158,400		$ 165,600
Non-Life Run-Off
Effects of Reinsurance [Line Items]
Increase (decrease) in reinsurance recoverable	$ (128,900)