Maximum Potential Amount of Future Payments, Credit Rating and Maturity Dates for Credit Default Swaps (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Credit Derivatives [Line Items]
Maximum Potential Amount of Future Payments	$ 230,000	$ 210,000
Corporate securities
Credit Derivatives [Line Items]
Maximum Potential Amount of Future Payments	$ 120,000	$ 120,000
Corporate securities | Minimum
Credit Derivatives [Line Items]
Maturity Date Range	2017-06	2017-06
Corporate securities | Maximum
Credit Derivatives [Line Items]
Maturity Date Range	2021-12	2020-12
Corporate securities | Credit Rating
Credit Derivatives [Line Items]
Credit Rating	AA-A	A
Sovereign Debt Securities
Credit Derivatives [Line Items]
Maximum Potential Amount of Future Payments	$ 110,000	$ 90,000
Sovereign Debt Securities | Minimum
Credit Derivatives [Line Items]
Maturity Date Range	2017-06	2017-06
Sovereign Debt Securities | Maximum
Credit Derivatives [Line Items]
Maturity Date Range	2021-12	2021-12
Sovereign Debt Securities | Credit Rating
Credit Derivatives [Line Items]
Credit Rating	AA-A	AA-A