Fair Value Part 3 (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 73,666,000		$ 42,188,000
Net LHFI	5,397,496,000		5,514,016,000
FDIC indemnification asset	20,198,000	28,260,000	21,774,000	28,348,000
Deposits	9,909,431,000		7,896,517,000
Long-term FHLB advances	10,969,000		0
Subordinated notes	49,879,000		49,871,000
Junior subordinated debt securities	94,856,000		61,856,000
Nonfinancial Assets and Liabilities [Abstract]
Outstanding balances in impaired loans	33,000,000		40,600,000
Foreclosed assets measured at fair value	50,000,000	8,900,000
Allowance for possible loan losses, Charge-offs	1,300,000	392,000
Foreclosed assets re-measured after initial recognition	13,200,000	13,200,000
Write-downs of allowance for foreclosed assets after initial recognition	1,600,000	2,400,000
Estimated fair value of financial instruments with immediate and shorter-term maturities maximum (in days)	90 days or less
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	248,822,000		238,535,000
Securities held to maturity	73,666,000		42,188,000
Deposits	9,909,431,000		7,896,517,000
Short-term liabilities	266,094,000		375,749,000
Long-term FHLB advances	10,969,000		0
Subordinated notes	49,879,000		49,871,000
Junior subordinated debt securities	94,856,000		61,856,000
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	248,822,000		238,535,000
Securities held to maturity	78,096,000		46,888,000
Deposits	9,918,637,000		7,904,179,000
Short-term liabilities	266,094,000		375,749,000
Long-term FHLB advances	10,969,000		0
Subordinated notes	54,195,000		53,980,000
Junior subordinated debt securities	75,465,000		40,206,000
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	5,397,496,000		5,514,016,000
Net acquired loans	1,044,258,000		127,489,000
FDIC indemnification asset	20,198,000		21,774,000
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	5,493,683,000		5,619,933,000
Net acquired loans	1,044,258,000		127,489,000
FDIC indemnification asset	$ 20,198,000		$ 21,774,000