Age of Gross Unrealized Losses and Associated Fair Value by Investment Category (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	$ 284,021	$ 284,996
Less than 12 Months Unrealized Loss	(683)	(2,562)
12 Months or More Fair Value	3,566	3,547
12 Months or More Unrealized Loss	(146)	(183)
Total Fair Value	287,587	288,543
Total Unrealized Loss	(829)	(2,745)
U.S. Treasuries
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value		2,334
Less than 12 Months Unrealized Loss		(18)
12 Months or More Fair Value		0
12 Months or More Unrealized Loss		0
Total Fair Value		2,334
Total Unrealized Loss		(18)
Mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	281,769	280,445
Less than 12 Months Unrealized Loss	(682)	(2,538)
12 Months or More Fair Value	1,317	1,336
12 Months or More Unrealized Loss	(94)	(92)
Total Fair Value	283,086	281,781
Total Unrealized Loss	(776)	(2,630)
Obligations of state and other political subdivisions
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	0	0
Less than 12 Months Unrealized Loss	0	0
12 Months or More Fair Value	2,232	2,194
12 Months or More Unrealized Loss	(52)	(91)
Total Fair Value	2,232	2,194
Total Unrealized Loss	(52)	(91)
Other securities
Investments, Unrealized Loss Position [Line Items]
Less than 12 Months Fair Value	2,252	2,217
Less than 12 Months Unrealized Loss	(1)	(6)
12 Months or More Fair Value	17	17
12 Months or More Unrealized Loss	0	0
Total Fair Value	2,269	2,234
Total Unrealized Loss	$ (1)	$ (6)