Available for Sale Investments' Gross Unrealized Losses and Fair Value, Aggregated by Investment Category and Length of Time that Individual Securities have been in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	$ 189,130	$ 351,462
Less than 12 months, Unrealized Losses	(551)	(1,142)
12 months or more, Fair Value	1,917	2,571
12 months or more, Unrealized Losses	(1)	(5)
Total Fair Value	191,047	354,033
Total Unrealized Losses	(552)	(1,147)
U.S. Treasury
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	10,141
Less than 12 months, Unrealized Losses	(36)
Total Fair Value	10,141
Total Unrealized Losses	(36)
U.S. Agencies
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	35,284	66,992
Less than 12 months, Unrealized Losses	(13)	(62)
Total Fair Value	35,284	66,992
Total Unrealized Losses	(13)	(62)
Mortgage-backed
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	57,372	226,081
Less than 12 months, Unrealized Losses	(202)	(919)
Total Fair Value	57,372	226,081
Total Unrealized Losses	(202)	(919)
State and political subdivisions
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	58,622	45,918
Less than 12 months, Unrealized Losses	(268)	(139)
12 months or more, Fair Value	1,917	2,571
12 months or more, Unrealized Losses	(1)	(5)
Total Fair Value	60,539	48,489
Total Unrealized Losses	(269)	(144)
Corporates
Gain (Loss) on Investments [Line Items]
Less than 12 months, Fair Value	27,711	12,471
Less than 12 months, Unrealized Losses	(32)	(22)
Total Fair Value	27,711	12,471
Total Unrealized Losses	$ (32)	$ (22)