Disclosures About Fair Value of Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	$ 916,423	$ 912,371
Level 1
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	2,924	3,157
Total financial assets	319,778	262,486
Level 2
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	903,193	898,617
Interest rate swaps	(6,205)	(6,273)
Total financial assets	996,182	1,004,909
Level 3
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	10,306	10,597
Total financial assets	6,333,554	6,250,676
Measured on recurring basis
Assets and liabilities measured at fair value on a recurring basis
Total financial assets	910,218	906,098
Measured on recurring basis | Equity securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	2,924	3,157
Measured on recurring basis | Debt securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	451,836	424,102
Measured on recurring basis | Debt securities | U.S. government and agencies
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	23	25
Measured on recurring basis | Debt securities | Debt issued by government sponsored enterprises
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	364,252	333,505
Measured on recurring basis | Debt securities | Municipal securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	67,279	70,145
Measured on recurring basis | Debt securities | Corporate debt issues
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	20,282	20,427
Measured on recurring basis | Mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	461,663	485,112
Measured on recurring basis | Residential mortgage-backed securities | GNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	28,192	29,216
Measured on recurring basis | Residential mortgage-backed securities | FNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	70,503	73,497
Measured on recurring basis | Residential mortgage-backed securities | FHLMC
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	39,611	42,119
Measured on recurring basis | Residential mortgage-backed securities | Non-agency CMOs
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	626	643
Measured on recurring basis | Collateralized mortgage obligations | GNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	7,679	8,329
Measured on recurring basis | Collateralized mortgage obligations | FNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	132,706	139,150
Measured on recurring basis | Collateralized mortgage obligations | FHLMC
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	169,614	178,698
Measured on recurring basis | Collateralized mortgage obligations | SBA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	9,419	10,052
Measured on recurring basis | Collateralized mortgage obligations | Non-agency CMOs
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	3,313	3,408
Measured on recurring basis | Interest rate swap agreements
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps	(6,205)	(6,273)
Measured on recurring basis | Level 1
Assets and liabilities measured at fair value on a recurring basis
Total financial assets	2,924	3,157
Measured on recurring basis | Level 1 | Equity securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	2,924	3,157
Measured on recurring basis | Level 2
Assets and liabilities measured at fair value on a recurring basis
Total financial assets	896,988	892,344
Measured on recurring basis | Level 2 | Debt securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	441,530	413,505
Measured on recurring basis | Level 2 | Debt securities | U.S. government and agencies
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	23	25
Measured on recurring basis | Level 2 | Debt securities | Debt issued by government sponsored enterprises
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	364,252	333,505
Measured on recurring basis | Level 2 | Debt securities | Municipal securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	67,279	70,145
Measured on recurring basis | Level 2 | Debt securities | Corporate debt issues
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	9,976	9,830
Measured on recurring basis | Level 2 | Mortgage-backed securities
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	461,663	485,112
Measured on recurring basis | Level 2 | Residential mortgage-backed securities | GNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	28,192	29,216
Measured on recurring basis | Level 2 | Residential mortgage-backed securities | FNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	70,503	73,497
Measured on recurring basis | Level 2 | Residential mortgage-backed securities | FHLMC
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	39,611	42,119
Measured on recurring basis | Level 2 | Residential mortgage-backed securities | Non-agency CMOs
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	626	643
Measured on recurring basis | Level 2 | Collateralized mortgage obligations | GNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	7,679	8,329
Measured on recurring basis | Level 2 | Collateralized mortgage obligations | FNMA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	132,706	139,150
Measured on recurring basis | Level 2 | Collateralized mortgage obligations | FHLMC
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	169,614	178,698
Measured on recurring basis | Level 2 | Collateralized mortgage obligations | SBA
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	9,419	10,052
Measured on recurring basis | Level 2 | Collateralized mortgage obligations | Non-agency CMOs
Assets and liabilities measured at fair value on a recurring basis
Mortgage-backed securities	3,313	3,408
Measured on recurring basis | Level 2 | Interest rate swap agreements
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps	(6,205)	(6,273)
Measured on recurring basis | Level 3
Assets and liabilities measured at fair value on a recurring basis
Total financial assets	10,306	10,597
Measured on recurring basis | Level 3 | Debt securities
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	10,306	10,597
Measured on recurring basis | Level 3 | Debt securities | Corporate debt issues
Assets and liabilities measured at fair value on a recurring basis
Securities available-for-sale	$ 10,306	$ 10,597