Fair Values of Assets and Liabilities - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			$ 1,169,328
Estimated Fair Value	1,297,203		1,171,303
Other investments	28,357		28,228
Loans Held for Sale			0
Other Assets			16,480
Total Assets			1,216,011
Other Liabilities			18,726	[1]
Total Liabilities			18,726
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value			1,975
Total Assets	1,420
US Government Agencies Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			31,664
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			864,401
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			149
Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			242,664
Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			86
Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			6,991
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities			23,373
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Estimated Fair Value	513		555
Other investments	0		0
Loans Held for Sale			0
Other Assets	0		0
Total Assets	513		555
Other Liabilities	0	[1]	0	[1]
Total Liabilities	0		0
Level 1 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	513		555
Level 1 [Member] | US Government Agencies Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 1 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 1 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 1 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 1 [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 1 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	1,270,758		1,145,955
Estimated Fair Value	1,270,758		1,145,955
Other investments	28,357		28,228
Loans Held for Sale			0
Other Assets	16,763		16,480
Total Assets	1,315,878		1,190,663
Other Liabilities	17,086	[1]	18,726	[1]
Total Liabilities	17,086		18,726
Level 2 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0		0
Level 2 [Member] | US Government Agencies Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	30,462		31,664
Level 2 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	990,660		864,401
Level 2 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	140		149
Level 2 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	242,438		242,664
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	85		86
Level 2 [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	6,973		6,991
Level 2 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	24,512		23,373
Estimated Fair Value	25,932		24,793
Other investments	0		0
Loans Held for Sale			0
Other Assets	0		0
Total Assets	25,932		24,793
Other Liabilities	0	[1]	0	[1]
Total Liabilities	0		0
Level 3 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,420		1,420
Level 3 [Member] | US Government Agencies Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	0		0
Level 3 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	24,512		23,373
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	1,295,270
Estimated Fair Value	1,297,203
Other investments	28,357
Other Assets	16,763
Total Assets	1,342,323
Other Liabilities	17,086	[1]
Total Liabilities	17,086
Estimate of Fair Value, Fair Value Disclosure [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,933
Estimate of Fair Value, Fair Value Disclosure [Member] | US Government Agencies Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	30,462
Estimate of Fair Value, Fair Value Disclosure [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	990,660
Estimate of Fair Value, Fair Value Disclosure [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	140
Estimate of Fair Value, Fair Value Disclosure [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	242,438
Estimate of Fair Value, Fair Value Disclosure [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	85
Estimate of Fair Value, Fair Value Disclosure [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	6,973
Estimate of Fair Value, Fair Value Disclosure [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Debt Securities	$ 24,512

[1] Non-hedging interest rate derivatives