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) - Decline 20% - 40% $ in Thousands		Sep. 30, 2018 USD ($) Investment	Dec. 31, 2017 USD ($) Investment
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 2,687	$ 2,033
Gross Unrealized Losses/OTTI	[1]	$ (1,029)	$ (908)
Number of Securities | Investment		3	2
Longer than six months but less than or equal to one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 518
Gross Unrealized Losses/OTTI	[1]	$ (256)
Number of Securities | Investment		1
Longer than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value		$ 2,169	$ 2,033
Gross Unrealized Losses/OTTI	[1]	$ (773)	$ (908)
Number of Securities | Investment		2	2

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