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, 2018	Dec. 31, 2017
Credit Derivatives [Line Items]
Maximum Potential Amount of Future Payments	$ 175,000	$ 185,000
Corporate securities
Credit Derivatives [Line Items]
Maximum Potential Amount of Future Payments	$ 95,000	$ 95,000
Corporate securities | Minimum
Credit Derivatives [Line Items]
Maturity Date Range	2020-03	2020-03
Corporate securities | Maximum
Credit Derivatives [Line Items]
Maturity Date Range	2021-12	2021-12
Corporate securities | Credit Rating
Credit Derivatives [Line Items]
Credit Rating	AA-BBB	AA-BBB
Sovereign Debt Securities
Credit Derivatives [Line Items]
Maximum Potential Amount of Future Payments	$ 80,000	$ 90,000
Sovereign Debt Securities | Minimum
Credit Derivatives [Line Items]
Maturity Date Range	2019-12	2018-03
Sovereign Debt Securities | Maximum
Credit Derivatives [Line Items]
Maturity Date Range	2022-06	2022-06
Sovereign Debt Securities | Credit Rating
Credit Derivatives [Line Items]
Credit Rating	AA-A	AA-A