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	Jun. 30, 2017 USD ($) Investment		Dec. 31, 2016 USD ($) Investment
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	$ 113,560		$ 228,354
Decline 20% - 40%
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	1,195		1,375
Gross Unrealized Losses/OTTI	$ (794)	[1]	$ (614)	[2]
Number of Securities | Investment	1		1
Decline 20% - 40% | Greater than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	$ 1,195		$ 1,375
Gross Unrealized Losses/OTTI	$ (794)	[1]	$ (614)	[2]
Number of Securities | Investment	1		1

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

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