Fair Value, Gross Unrealized Losses, Other-Than-Temporary Impairments Recognized in Other Comprehensive Income and Number of Securities with Fair Value Declining below Amortized Cost (Detail) $ in Thousands		Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 191,386	$ 228,354
Decline 20% - 40%
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		2,033	1,375
Gross Unrealized Losses/OTTI	[1]	$ (908)	$ (614)
Number of Securities | Investment		2	1
Decline 20% - 40% | Longer than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 2,033	$ 1,375
Gross Unrealized Losses/OTTI	[1]	$ (908)	$ (614)
Number of Securities | Investment		2	1

[1] Subsequent unrealized gains (losses) on OTTI securities are included in Net unrealized OTTI on securities in the Statements of Comprehensive Income (Loss).