Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 Security	Mar. 31, 2012	Dec. 31, 2012
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	$ 1,074,179		$ 905,846
Gross Unrealized Gains	5,038		6,903
Gross Unrealized Losses	1,733		657
Total securities available for sale	1,077,484		912,092
Amortized Cost [Abstract]
Due in one year or less	5,599
Due in one year through five years	921,922
Due after five years through ten years	140,534
Due after ten years	6,114
Amortized Cost	1,074,169
Fair Value [Abstract]
Due in one year or less	5,704
Due in one year through five years	924,765
Due after five years through ten years	140,678
Due after ten years	6,327
Available for sale debt securities, fair value	1,077,474
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	404,199		204,463
12 months or more - Fair Value	0		0
Total - Fair Value	404,199		204,463
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	1,733		657
12 months or more - Gross Unrealized Loss	0		0
Total - Gross Unrealized Loss	1,733		657
Proceeds from sales and calls of securities available for sale, gross realized gains and gross realized losses [Abstract]
Proceeds from sales	0	37,857
Proceeds from calls	108,997	234,725
Gross realized gains	2	775
Gross realized losses	0	98
Income tax expense recognized on net gains on sales and calls of securities available for sale	1	237
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost	117,973		143,426
Gross Unrecognized Gains	7,107		7,700
Gross Unrecognized Losses	0		0
Fair Value	125,080	151,126	151,126
Amortized Cost [Abstract]
Due in one year or less	10,002
Due in one year through five years	107,971
Due in five years through ten years	0
Amortized Cost	117,973
Fair Value [Abstract]
Due in one year or less	10,055
Due in one year through five years	115,025
Due in five years through ten years	0
Fair Value	125,080
Investment security impairment - Other Than Temporary Impairment [Abstract]
Number of securities in portfolio	253
Number of securities in portfolio in unrealized loss position	46
Mortgage-backed securities and collateralized mortgage obligations-residential [Member]
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost	98,038		108,471
Gross Unrecognized Gains	5,377		5,724
Gross Unrecognized Losses	0		0
Fair Value	103,415		114,195
Corporate bonds [Member]
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost	19,935		34,955
Gross Unrecognized Gains	1,730		1,976
Gross Unrecognized Losses	0		0
Fair Value	21,665		36,931
U. S. government sponsored enterprises [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	262,565		262,063
Gross Unrealized Gains	665		1,055
Gross Unrealized Losses	65		10
Total securities available for sale	263,165		263,108
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	55,838		15,491
12 months or more - Fair Value	0		0
Total - Fair Value	55,838		15,491
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	65		10
12 months or more - Gross Unrealized Loss	0		0
Total - Gross Unrealized Loss	65		10
State and political subdivisions [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	14,743		25,815
Gross Unrealized Gains	522		642
Gross Unrealized Losses	0		0
Total securities available for sale	15,265		26,457
Mortgage-backed securities and collateralized mortgage obligations-residential [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	610,632		515,322
Gross Unrealized Gains	3,201		3,982
Gross Unrealized Losses	1,278		528
Total securities available for sale	612,555		518,776
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	261,302		178,689
12 months or more - Fair Value	0		0
Total - Fair Value	261,302		178,689
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	1,278		528
12 months or more - Gross Unrealized Loss	0		0
Total - Gross Unrealized Loss	1,278		528
Corporate bonds [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	58,802		26,312
Gross Unrealized Gains	440		336
Gross Unrealized Losses	3		119
Total securities available for sale	59,239		26,529
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	5,614		10,283
12 months or more - Fair Value	0		0
Total - Fair Value	5,614		10,283
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	3		119
12 months or more - Gross Unrealized Loss	0		0
Total - Gross Unrealized Loss	3		119
Small Business Administration-guaranteed participation securities [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	115,615		75,674
Gross Unrealized Gains	210		888
Gross Unrealized Losses	361		0
Total securities available for sale	115,464		76,562
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	70,309
12 months or more - Fair Value	0
Total - Fair Value	70,309
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	361
12 months or more - Gross Unrealized Loss	0
Total - Gross Unrealized Loss	361
Mortgage-backed securities and collateralized mortgage obligations-Commercial [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	11,162
Gross Unrealized Gains	0
Gross Unrealized Losses	26
Total securities available for sale	11,136
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	11,136
12 months or more - Fair Value	0
Total - Fair Value	11,136
Available for sale securities unrealized loss position - Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	26
12 months or more - Gross Unrealized Loss	0
Total - Gross Unrealized Loss	26
Other [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	650		650
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Total securities available for sale	650		650
Debt securities [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	1,074,169		905,836
Gross Unrealized Gains	5,038		6,903
Gross Unrealized Losses	1,733		657
Total securities available for sale	1,077,474		912,082
Equity securities [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	10		10
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Total securities available for sale	$ 10		$ 10