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	Jun. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 1,306,687		$ 1,171,303
Other investments	33,913		28,228
Loans Held for Sale	0		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]
Total Assets	1,420
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	688		555
Other investments	0		0
Loans Held for Sale	0		0
Other Assets	0		0
Total Assets	688		555
Other Liabilities	0	[1]	0	[1]
Total Liabilities	0		0
Level 1 [Member] | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0		0
Level 1 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	688		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]
Estimated Fair Value	0		0
Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0		0
Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0		0
Level 1 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	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]
Estimated Fair Value	0		0
Level 1 [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	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]
Estimated Fair Value	0		0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,278,710		1,145,955
Other investments	33,913		28,228
Loans Held for Sale	0		0
Other Assets	14,121		16,480
Total Assets	1,326,744		1,190,663
Other Liabilities	14,363	[1]	18,726	[1]
Total Liabilities	14,363		18,726
Level 2 [Member] | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,278,710		1,145,955
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]
Estimated Fair Value	28,036		31,664
Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	978,185		864,401
Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	126		149
Level 2 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	265,296		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]
Estimated Fair Value	83		86
Level 2 [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	6,984		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]
Estimated Fair Value	0		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	27,289		24,793
Other investments	0		0
Loans Held for Sale	0		0
Other Assets	0		0
Total Assets	27,289		24,793
Other Liabilities	0	[1]	0	[1]
Total Liabilities	0		0
Level 3 [Member] | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	25,869		23,373
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]
Estimated Fair Value	0		0
Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0		0
Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0		0
Level 3 [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	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]
Estimated Fair Value	0		0
Level 3 [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	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]
Estimated Fair Value	25,869		23,373
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,306,687
Other investments	33,913
Other Assets	14,121
Total Assets	1,354,721
Other Liabilities	14,363	[1]
Total Liabilities	14,363
Estimate of Fair Value, Fair Value Disclosure [Member] | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	1,304,579		1,169,328
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	2,108		1,975
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]
Estimated Fair Value	28,036		31,664
Estimate of Fair Value, Fair Value Disclosure [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Residential [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	978,185		864,401
Estimate of Fair Value, Fair Value Disclosure [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Mortgage Backed Securities Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	126		149
Estimate of Fair Value, Fair Value Disclosure [Member] | US Government-sponsored Enterprises Debt Securities [Member] | Other Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	265,296		242,664
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]
Estimated Fair Value	83		86
Estimate of Fair Value, Fair Value Disclosure [Member] | Corporate securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	6,984		6,991
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]
Estimated Fair Value	$ 25,869		$ 23,373

[1] Non-hedging interest rate derivatives