Securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 Position	Mar. 31, 2012	Dec. 31, 2012 Position
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,441,073,000		$ 1,120,384,000
Unrealized gains	25,293,000		27,794,000
Unrealized losses	575,000		179,000
Estimated fair value	1,465,791,000		1,147,999,000
Sales transactions of securities available for sale [Abstract]
Proceeds from sales	2,600,000	1,800,000
Gross realized gains	1,100,000	500,000
Amortized costs of securities available for sale pledged to secure public deposits	1,500,000,000		1,200,000,000
Amortized costs of securities available for sale pledged as collateral for repurchase agreements	236,600,000		209,000,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	225,626,000		40,397,000
12 Months or Longer, Fair Value	186,000		167,000
Total, Fair Value	225,812,000		40,564,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	(513,000)		(97,000)
12 Months or Longer, Unrealized Losses	(62,000)		(82,000)
Total, Unrealized Losses	(575,000)		(179,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	103		7
12 Months or Longer, Number of Positions	1		1
Total, Number of Positions	104		8
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	62,474,000		60,563,000
Unrealized gains	887,000		972,000
Unrealized losses	0		0
Estimated fair value	63,361,000		61,535,000
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Within one year	45,889,000
From one to five years	184,744,000
From five to ten years	382,001,000
After ten years	815,014,000
Total, Amortized Cost	1,427,648,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Within one year	46,070,000
From one to five years	187,836,000
From five to ten years	388,831,000
After ten years	827,199,000
Total, Fair Value	1,449,936,000
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Within one year	24,547,000
From one to five years	28,732,000
From five to ten years	6,125,000
After ten years	3,070,000
Securities held to maturity	62,474,000
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Within one year	24,598,000
From one to five years	29,380,000
From five to ten years	6,130,000
After ten years	3,253,000
Total, Fair Value	63,361,000
Consolidated stockholders' equity threshold percentage that no single issuer of securities exceeded except for U.S. Government securities. (in hundredths)	10.00%
State & Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	61,347,000		59,395,000
Unrealized gains	704,000		788,000
Unrealized losses	0		0
Estimated fair value	62,051,000		60,183,000
Mortgage-backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,127,000		1,168,000
Unrealized gains	183,000		184,000
Unrealized losses	0		0
Estimated fair value	1,310,000		1,352,000
US Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	68,557,000		63,668,000
Unrealized gains	668,000		757,000
Unrealized losses	0		0
Estimated fair value	69,225,000		64,425,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	0		0
12 Months or Longer, Fair Value	0		0
Total, Fair Value	0		0
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	0		0
12 Months or Longer, Unrealized Losses	0		0
Total, Unrealized Losses	0		0
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	0		0
Total, Number of Positions	0		0
Federal Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	296,135,000		281,398,000
Unrealized gains	1,343,000		1,507,000
Unrealized losses	105,000		91,000
Estimated fair value	297,373,000		282,814,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	39,955,000		39,906,000
12 Months or Longer, Fair Value	0		0
Total, Fair Value	39,955,000		39,906,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	(105,000)		(91,000)
12 Months or Longer, Unrealized Losses	0		0
Total, Unrealized Losses	(105,000)		(91,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	6		4
12 Months or Longer, Number of Positions	0		0
Total, Number of Positions	6		4
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	15,977,000
12 Months or Longer, Fair Value	0
Total, Fair Value	15,977,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	(27,000)
12 Months or Longer, Unrealized Losses	0
Total, Unrealized Losses	(27,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	11
12 Months or Longer, Number of Positions	0
Total, Number of Positions	11
Mortgage-Backed [Member] | Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	304,955,000		221,110,000
Unrealized gains	10,130,000		11,175,000
Unrealized losses	25,000		0
Estimated fair value	315,060,000		232,285,000
Mortgage-Backed [Member] | U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	28,649,000		16,351,000
Unrealized gains	1,641,000		1,645,000
Unrealized losses	2,000		0
Estimated fair value	30,288,000		17,996,000
State & Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	142,714,000		82,675,000
Unrealized gains	3,579,000		4,127,000
Unrealized losses	41,000		0
Estimated fair value	146,252,000		86,802,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	20,335,000
12 Months or Longer, Fair Value	0
Total, Fair Value	20,335,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	(41,000)
12 Months or Longer, Unrealized Losses	0
Total, Unrealized Losses	(41,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	68
12 Months or Longer, Number of Positions	0
Total, Number of Positions	68
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	149,359,000		23,000
12 Months or Longer, Fair Value	0		0
Total, Fair Value	149,359,000		23,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	(340,000)		0
12 Months or Longer, Unrealized Losses	0		0
Total, Unrealized Losses	(340,000)		0
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	18		2
12 Months or Longer, Number of Positions	0		0
Total, Number of Positions	18		2
Collateralized Mortgage Obligations [Member] | Government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	526,545,000		399,147,000
Unrealized gains	4,247,000		4,418,000
Unrealized losses	339,000		0
Estimated fair value	530,453,000		403,565,000
Collateralized Mortgage Obligations [Member] | U.S. government securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	60,093,000		44,825,000
Unrealized gains	1,193,000		1,333,000
Unrealized losses	1,000		0
Estimated fair value	61,285,000		46,158,000
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	13,425,000		11,210,000
Unrealized gains	2,492,000		2,832,000
Unrealized losses	62,000		88,000
Estimated fair value	15,855,000		13,954,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	0		468,000
12 Months or Longer, Fair Value	186,000		167,000
Total, Fair Value	186,000		635,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses	0		(6,000)
12 Months or Longer, Unrealized Losses	(62,000)		(82,000)
Total, Unrealized Losses	$ (62,000)		$ (88,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	0		1
12 Months or Longer, Number of Positions	1		1
Total, Number of Positions	1		2