Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 10,331	$ 6,176
Less than 12 Months Gross Unrealized Losses	(55)	(16)
12 Months or Longer Fair Value	171	163
12 Months or Longer Gross Unrealized Losses	(5,806)	(5,572)
Total Fair Value	10,502	6,339
Total Gross Unrealized Losses	(5,861)	(5,588)
State and municipal
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	3,868
Less than 12 Months Gross Unrealized Losses	(25)
Total Fair Value	3,868
Total Gross Unrealized Losses	(25)
U.S. Government-sponsored mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	6,463	6,176
Less than 12 Months Gross Unrealized Losses	(30)	(16)
Total Fair Value	6,463	6,176
Total Gross Unrealized Losses	(30)	(16)
Corporate obligations
Investments, Unrealized Loss Position [Line Items]
12 Months or Longer Fair Value	171	163
12 Months or Longer Gross Unrealized Losses	(5,806)	(5,572)
Total Fair Value	171	163
Total Gross Unrealized Losses	$ (5,806)	$ (5,572)