Gross Unrealized Losses and Fair Value, Aggregated by Investment Category and Length of Time that Individual Securities have been in a Continuous Unrealized Loss Position (Detail) - U.S. government agencies - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Gain (Loss) on Investments [Line Items]
Less than 12 Months, Fair Value	$ 4,998	$ 1,141
Less than 12 Months, Unrealized Losses	(2)	(2)
12 Months or More, Fair Value	0	5,935
12 Months or More, Unrealized Losses	0	(65)
Total, Fair Value	4,998	7,076
Total, Unrealized Losses	$ (2)	$ (67)