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	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	$ 1,308,399	$ 1,316,945
Estimated Fair Value of Available for Sale	1,309,819	1,318,365
Other investments	44,545	35,444
Loans Held for Sale	2,502	0
Other Assets	11,186	14,358
Total Assets	1,368,052	1,368,167
Other Liabilities	10,671	14,318
Total Liabilities	10,671	14,318
Equities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value of Available for Sale	1,420	1,420
US Government Agencies Debt Securities [Member] | Mortgage-Backed Securities-Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	25,936	25,204
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage-Backed Securities-Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	950,881	994,887
Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage-Backed Securities-Commercial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	74	105
Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government - Sponsored Enterprises [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	267,877	266,125
Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	27,377	80
Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	7,255	7,021
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	28,999	23,523
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Estimated Fair Value of Available for Sale	0	0
Other investments	0	0
Loans Held for Sale	0	0
Other Assets	0	0
Total Assets	0	0
Other Liabilities	0	0
Total Liabilities	0	0
Level 1 [Member] | Equities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value of Available for Sale	0	0
Level 1 [Member] | US Government Agencies Debt Securities [Member] | Mortgage-Backed Securities-Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 1 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 1 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 1 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	1,279,400	1,293,422
Estimated Fair Value of Available for Sale	1,279,400	1,293,422
Other investments	44,545	35,444
Loans Held for Sale	2,502	0
Other Assets	11,186	14,358
Total Assets	1,337,633	1,343,224
Other Liabilities	10,671	14,318
Total Liabilities	10,671	14,318
Level 2 [Member] | Equities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value of Available for Sale	0	0
Level 2 [Member] | US Government Agencies Debt Securities [Member] | Mortgage-Backed Securities-Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	25,936	25,204
Level 2 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage-Backed Securities-Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	950,881	994,887
Level 2 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Mortgage-Backed Securities-Commercial [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	74	105
Level 2 [Member] | Obligations of U.S. Government-Sponsored Enterprises [Member] | Other Government - Sponsored Enterprises [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	267,877	266,125
Level 2 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	27,377	80
Level 2 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	7,255	7,021
Level 2 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	28,999	23,523
Estimated Fair Value of Available for Sale	30,419	24,943
Other investments	0	0
Loans Held for Sale	0	0
Other Assets		0
Total Assets	30,419	24,943
Other Liabilities	0	0
Total Liabilities	0	0
Level 3 [Member] | Equities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Estimated Fair Value of Available for Sale		1,420
Level 3 [Member] | US Government Agencies Debt Securities [Member] | Mortgage-Backed Securities-Residential [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [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, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 3 [Member] | Obligations of States and Political Subdivisions [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 3 [Member] | Corporate Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	0	0
Level 3 [Member] | Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total Debt Securities	$ 28,999	$ 23,523