Securities (Details) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2013 Position	Jun. 30, 2013 Position	Jun. 30, 2012	Dec. 31, 2012 Position
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,390,336,000	$ 1,390,336,000		$ 1,120,384,000
Unrealized gains		16,861,000		27,794,000
Unrealized losses		16,794,000		179,000
Estimated fair value	1,390,403,000	1,390,403,000		1,147,999,000
Sales transactions of securities available for sale [Abstract]
Proceeds from sales		26,200,000	1,800,000
Gross realized gains		1,100,000	600,000
Amortized costs of securities available for sale pledged to secure public deposits	1,500,000,000	1,500,000,000		1,200,000,000
Amortized costs of securities available for sale pledged as collateral for repurchase agreements	224,600,000	224,600,000		209,000,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	1,328,996,000	1,328,996,000		40,397,000
12 Months or Longer, Fair Value	211,000	211,000		167,000
Total, Fair Value	1,329,207,000	1,329,207,000		40,564,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(16,757,000)		(97,000)
12 Months or Longer, Unrealized Losses		(37,000)		(82,000)
Total, Unrealized Losses		(16,794,000)		(179,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	337	337		7
12 Months or Longer, Number of Positions	1	1		1
Total, Number of Positions	338	338		8
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	122,302,000	122,302,000		60,563,000
Unrealized gains	731,000			972,000
Unrealized losses	1,964,000			0
Total, Fair Value	121,069,000	121,069,000		61,535,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	64,022,000	64,022,000
12 Months or Longer, Fair Value	0	0
Total, Fair Value	64,022,000	64,022,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(1,964,000)
12 Months or Longer, Unrealized Losses		0
Total, Unrealized Losses		(1,964,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	5	5
12 Months or Longer, Number of Positions	0	0
Total, Number of Positions	5	5
Consolidated stockholders' equity threshold percentage that no single issuer of securities exceeded except for U.S. Government securities. (in hundredths)	10.00%	10.00%
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Within one year	26,789,000	26,789,000
From one to five years	194,833,000	194,833,000
From five to ten years	348,099,000	348,099,000
After ten years	807,206,000	807,206,000
Total, Amortized Cost	1,376,927,000	1,376,927,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Within one year	26,915,000	26,915,000
From one to five years	195,558,000	195,558,000
From five to ten years	345,525,000	345,525,000
After ten years	806,445,000	806,445,000
Total, Fair Value	1,374,443,000	1,374,443,000
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Within one year	25,315,000	25,315,000
From one to five years	24,620,000	24,620,000
From five to ten years	5,651,000	5,651,000
After ten years	66,716,000	66,716,000
Securities held to maturity	122,302,000	122,302,000		60,563,000
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Within one year	25,415,000	25,415,000
From one to five years	25,095,000	25,095,000
From five to ten years	5,656,000	5,656,000
After ten years	64,903,000	64,903,000
Total, Fair Value	121,069,000	121,069,000		61,535,000
State & Municipal [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	57,210,000	57,210,000		59,395,000
Unrealized gains	580,000			788,000
Unrealized losses	0			0
Total, Fair Value	57,790,000	57,790,000		60,183,000
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Securities held to maturity	57,210,000	57,210,000		59,395,000
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	57,790,000	57,790,000		60,183,000
Mortgage backed [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	1,070,000	1,070,000		1,168,000
Unrealized gains	151,000			184,000
Unrealized losses	0			0
Total, Fair Value	1,221,000	1,221,000		1,352,000
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Securities held to maturity	1,070,000	1,070,000		1,168,000
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	1,221,000	1,221,000		1,352,000
Collateralized Mortgage Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	64,022,000	64,022,000		0
Unrealized gains	0			0
Unrealized losses	1,964,000			0
Total, Fair Value	62,058,000	62,058,000		0
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	64,022,000	64,022,000
12 Months or Longer, Fair Value	0	0
Total, Fair Value	64,022,000	64,022,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(1,964,000)
12 Months or Longer, Unrealized Losses		0
Total, Unrealized Losses		(1,964,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	5	5
12 Months or Longer, Number of Positions	0	0
Total, Number of Positions	5	5
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Securities held to maturity	64,022,000	64,022,000		0
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Total, Fair Value	62,058,000	62,058,000		0
U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost				16,351,000
Unrealized gains				1,645,000
Unrealized losses				0
Estimated fair value				17,996,000
US Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	53,440,000	53,440,000		63,668,000
Unrealized gains		509,000		757,000
Unrealized losses		0		0
Estimated fair value	53,949,000	53,949,000		64,425,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	0	0		0
12 Months or Longer, Fair Value	0	0		0
Total, Fair Value	0	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		0
12 Months or Longer, Number of Positions	0	0		0
Total, Number of Positions	0	0		0
Federal Agency [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	286,049,000	286,049,000		281,398,000
Unrealized gains		192,000		1,507,000
Unrealized losses		6,783,000		91,000
Estimated fair value	279,458,000	279,458,000		282,814,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	286,049,000	286,049,000		39,906,000
12 Months or Longer, Fair Value	0	0		0
Total, Fair Value	286,049,000	286,049,000		39,906,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(6,783,000)		(91,000)
12 Months or Longer, Unrealized Losses		0		0
Total, Unrealized Losses		(6,783,000)		(91,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	20	20		4
12 Months or Longer, Number of Positions	0	0		0
Total, Number of Positions	20	20		4
Mortgage-Backed [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	285,687,000	285,687,000		0
12 Months or Longer, Fair Value	0	0		0
Total, Fair Value	285,687,000	285,687,000		0
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(1,562,000)		0
12 Months or Longer, Unrealized Losses		0		0
Total, Unrealized Losses		(1,562,000)		0
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	94	94		0
12 Months or Longer, Number of Positions	0	0		0
Total, Number of Positions	94	94		0
Mortgage-Backed [Member] | Government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	260,130,000	260,130,000		221,110,000
Unrealized gains		6,149,000		11,175,000
Unrealized losses		1,487,000		0
Estimated fair value	264,792,000	264,792,000		232,285,000
Mortgage-Backed [Member] | U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	25,608,000	25,608,000
Unrealized gains		1,269,000
Unrealized losses		75,000
Estimated fair value	26,802,000	26,802,000
State & Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	125,792,000	125,792,000		82,675,000
Unrealized gains		2,375,000		4,127,000
Unrealized losses		1,493,000		0
Estimated fair value	126,674,000	126,674,000		86,802,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	125,792,000	125,792,000		0
12 Months or Longer, Fair Value	0	0		0
Total, Fair Value	125,792,000	125,792,000		0
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(1,493,000)		0
12 Months or Longer, Unrealized Losses		0		0
Total, Unrealized Losses		(1,493,000)		0
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	193	193		0
12 Months or Longer, Number of Positions	0	0		0
Total, Number of Positions	193	193		0
Collateralized Mortgage Obligations [Member]
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	625,908,000	625,908,000		23,000
12 Months or Longer, Fair Value	0	0		0
Total, Fair Value	625,908,000	625,908,000		23,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(6,787,000)		0
12 Months or Longer, Unrealized Losses		0		0
Total, Unrealized Losses		(6,787,000)		0
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	28	28		2
12 Months or Longer, Number of Positions	0	0		0
Total, Number of Positions	28	28		2
Collateralized Mortgage Obligations [Member] | Government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	573,487,000	573,487,000		399,147,000
Unrealized gains		2,677,000		4,418,000
Unrealized losses		6,747,000		0
Estimated fair value	569,417,000	569,417,000		403,565,000
Collateralized Mortgage Obligations [Member] | U.S. government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	52,421,000	52,421,000		44,825,000
Unrealized gains		970,000		1,333,000
Unrealized losses		40,000		0
Estimated fair value	53,351,000	53,351,000		46,158,000
Other Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	13,409,000	13,409,000		11,210,000
Unrealized gains		2,720,000		2,832,000
Unrealized losses		169,000		88,000
Estimated fair value	15,960,000	15,960,000		13,954,000
Unrealized Loss Position, Fair Value [Abstract]
Less Than 12 Months, Fair Value	5,560,000	5,560,000		468,000
12 Months or Longer, Fair Value	211,000	211,000		167,000
Total, Fair Value	5,771,000	5,771,000		635,000
Unrealized Loss Position, Unrealized Losses [Abstract]
Less Than 12 Months, Unrealized Losses		(132,000)		(6,000)
12 Months or Longer, Unrealized Losses		(37,000)		(82,000)
Total, Unrealized Losses		$ (169,000)		$ (88,000)
Unrealized Loss Position, Number of Positions [Abstract]
Less Than 12 Months, Number of Positions	2	2		1
12 Months or Longer, Number of Positions	1	1		1
Total, Number of Positions	3	3		2