Fair Value, Gross Unrealized Losses, Other-Than-Temporary Impairments Recognized in Other Comprehensive Income ("OCI") and Number of Securities with Fair Value Declining below Amortized Cost (Detail) $ in Thousands	Mar. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($) Investment
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	$ 165,057	$ 228,354
Decline 20% - 40%
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	1,423	1,375
Gross Unrealized Losses/OTTI	$ (566)	$ (614)
Number of Securities | Investment	1	1
Decline 20% - 40% | Greater than one year
Gain (Loss) on Investments [Line Items]
Estimated Fair Value	$ 1,423	$ 1,375
Gross Unrealized Losses/OTTI	$ (566)	$ (614)
Number of Securities | Investment	1	1