Investment Securities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	$ 1,113,030,000		$ 1,113,030,000		$ 919,415,000
Gross Unrealized Gains	6,270,000		6,270,000		4,732,000
Gross Unrealized Losses	1,245,000		1,245,000		6,425,000
Total securities available-for-sale	1,118,055,000		1,118,055,000		917,722,000
Amortized Cost [Abstract]
Due in one year or less	11,632,000		11,632,000
Due in one year through five years	824,740,000		824,740,000
Due after five years through ten years	250,078,000		250,078,000
Due after ten years	16,938,000		16,938,000
Amortized Cost	1,103,388,000		1,103,388,000
Fair Value [Abstract]
Due in one year or less	11,733,000		11,733,000
Due in one year through five years	828,350,000		828,350,000
Due after five years through ten years	250,771,000		250,771,000
Due after ten years	17,559,000		17,559,000
Fair Value	1,108,413,000		1,108,413,000
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	48,929,000		48,929,000		327,327,000
12 months or more - Fair Value	41,011,000		41,011,000		20,248,000
Total - Fair Value	89,940,000		89,940,000		347,575,000
Available for sale securities unrealized loss position- Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	170,000		170,000		5,156,000
12 months or more - Gross Unrealized Loss	1,075,000		1,075,000		1,269,000
Total - Gross Unrealized Loss	1,245,000		1,245,000		6,425,000
Proceeds from sales and calls of securities available for sale, gross realized gains and gross realized losses [Abstract]
Proceeds from sales	16,141,000	40,427,000	53,998,000	45,976,000
Proceeds from calls	313,024,000	341,214,000	547,749,000	357,661,000
Gross realized gains	157,000	888,000	932,000	1,175,000
Gross realized losses	102,000	37,000	200,000	37,000
Income tax expense recognized on net gains on sales and calls of securities available for sale	22,000	340,000	293,000	455,000
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost	168,755,000		168,755,000		216,288,000
Gross Unrecognized Gains	8,603,000		8,603,000		8,580,000
Gross Unrecognized Losses	0		0		428,000
Fair Value	177,358,000		177,358,000		224,440,000
Amortized Cost [Abstract]
Due in one year or less	25,271,000		25,271,000
Due in one year through five years	115,652,000		115,652,000
Due in five years through ten years	27,832,000		27,832,000
Amortized Cost	168,755,000		168,755,000
Fair Value [Abstract]
Due in one year or less	25,822,000		25,822,000
Due in one year through five years	122,369,000		122,369,000
Due in five years through ten years	29,167,000		29,167,000
Fair Value	177,358,000		177,358,000
Held to maturity securities in an unrecognized loss position	0		0
Held to maturity unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value					28,860,000
12 months or more - Fair Value					0
Total - Fair Value					28,860,000
Held to maturity unrealized loss position - Gross Unrec. Loss [Abstract]
Less than 12 months - Gross Unrec. Loss					428,000
12 months or more - Gross Unrec. Loss					0
Total - Gross Unrec. Loss					428,000
Number of securities in portfolio	279		279
Number of securities in portfolio in unrealized loss position	14		14
U. S. government sponsored enterprises [Member]
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost					15,000,000
Gross Unrecognized Gains					19,000
Gross Unrecognized Losses					0
Fair Value					15,019,000
Mortgage-backed securities and collateralized mortgage obligations-residential [Member]
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost	133,562,000		133,562,000		141,857,000
Gross Unrecognized Gains	7,197,000		7,197,000		7,727,000
Gross Unrecognized Losses	0		0		46,000
Fair Value	140,759,000		140,759,000		149,538,000
Held to maturity unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value					19,328,000
12 months or more - Fair Value					0
Total - Fair Value					19,328,000
Held to maturity unrealized loss position - Gross Unrec. Loss [Abstract]
Less than 12 months - Gross Unrec. Loss					46,000
12 months or more - Gross Unrec. Loss					0
Total - Gross Unrec. Loss					46,000
Corporate bonds [Member]
Amortized cost and fair value of the held to maturity securities [Abstract]
Amortized Cost	35,193,000		35,193,000		59,431,000
Gross Unrecognized Gains	1,406,000		1,406,000		834,000
Gross Unrecognized Losses	0		0		382,000
Fair Value	36,599,000		36,599,000		59,883,000
Held to maturity unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value					9,532,000
12 months or more - Fair Value					0
Total - Fair Value					9,532,000
Held to maturity unrealized loss position - Gross Unrec. Loss [Abstract]
Less than 12 months - Gross Unrec. Loss					382,000
12 months or more - Gross Unrec. Loss					0
Total - Gross Unrec. Loss					382,000
U. S. government sponsored enterprises [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	641,497,000		641,497,000		562,588,000
Gross Unrealized Gains	1,692,000		1,692,000		1,171,000
Gross Unrealized Losses	0		0		300,000
Total securities available-for-sale	643,189,000		643,189,000		563,459,000
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value					147,881,000
12 months or more - Fair Value					0
Total - Fair Value					147,881,000
Available for sale securities unrealized loss position- Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss					300,000
12 months or more - Gross Unrealized Loss					0
Total - Gross Unrealized Loss					300,000
State and political subdivisions
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	36,046,000		36,046,000		42,812,000
Gross Unrealized Gains	932,000		932,000		1,156,000
Gross Unrealized Losses	0		0		0
Total securities available-for-sale	36,978,000		36,978,000		43,968,000
Mortgage-backed securities and collateralized mortgage obligations-residential [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	351,025,000		351,025,000		202,103,000
Gross Unrealized Gains	3,410,000		3,410,000		2,335,000
Gross Unrealized Losses	150,000		150,000		415,000
Total securities available-for-sale	354,285,000		354,285,000		204,023,000
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	38,314,000		38,314,000		107,369,000
12 months or more - Fair Value	0		0		781,000
Total - Fair Value	38,314,000		38,314,000		108,150,000
Available for sale securities unrealized loss position- Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	150,000		150,000		369,000
12 months or more - Gross Unrealized Loss	0		0		46,000
Total - Gross Unrealized Loss	150,000		150,000		415,000
Corporate bonds [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	74,170,000		74,170,000		102,248,000
Gross Unrealized Gains	236,000		236,000		70,000
Gross Unrealized Losses	1,095,000		1,095,000		5,710,000
Total securities available-for-sale	73,311,000		73,311,000		96,608,000
Available for sale securities unrealized loss position - Fair Value [Abstract]
Less than 12 months - Fair Value	10,615,000		10,615,000		72,077,000
12 months or more - Fair Value	41,011,000		41,011,000		19,467,000
Total - Fair Value	51,626,000		51,626,000		91,544,000
Available for sale securities unrealized loss position- Gross Unrealized Loss [Abstract]
Less than 12 months - Gross Unrealized Loss	20,000		20,000		4,487,000
12 months or more - Gross Unrealized Loss	1,075,000		1,075,000		1,223,000
Total - Gross Unrealized Loss	1,095,000		1,095,000		5,710,000
Other [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	650,000		650,000		650,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Total securities available-for-sale	650,000		650,000		650,000
Debt securities [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	1,103,388,000		1,103,388,000		910,401,000
Gross Unrealized Gains	6,270,000		6,270,000		4,732,000
Gross Unrealized Losses	1,245,000		1,245,000		6,425,000
Total securities available-for-sale	1,108,413,000		1,108,413,000		908,708,000
Equity securities [Member]
Amortized cost and fair value of the securities available for sale [Abstract]
Amortized Cost	9,642,000		9,642,000		9,014,000
Gross Unrealized Gains	0		0		0
Gross Unrealized Losses	0		0		0
Total securities available-for-sale	9,642,000		9,642,000		9,014,000
Federal Home Loan Bank stock and Federal Reserve Bank Stock included in equity securities	$ 9,600,000		$ 9,600,000		$ 9,000,000