Fair Value Part 3 (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 1,184,554,000		$ 1,170,685,000
Net LHFI	6,342,555,000		6,379,853,000
FDIC indemnification asset	4,743,000	13,487,000	6,997,000	14,347,000
Deposits	9,906,990,000		9,698,358,000
Long-term FHLB advances	1,222,000		1,253,000
Subordinated notes	49,944,000		49,936,000
Junior subordinated debt securities	61,856,000		61,856,000
Nonfinancial Assets and Liabilities [Abstract]
Outstanding balances in impaired loans	48,300,000		47,100,000
Foreclosed assets measured at fair value	8,700,000	13,700,000
Allowance for possible loan losses, Charge-offs for foreclosed assets upon initial recognition	2,300,000	5,100,000
Foreclosed assets re-measured after initial recognition	13,700,000	7,600,000
Write-downs of allowance for foreclosed assets after initial recognition	882,000	1,100,000
Estimated fair value of financial instruments with immediate and shorter-term maturities maximum	90 days
Noninterest income in Mortgage banking, net for changes in fair value of LHFS	382,000
Interest earned on LHFS included in Interest and fees on LHFS and LHFI	1,100,000
Fair value and the contractual principal outstanding of the LHFS [Abstract]
Fair value of LHFS	133,754,000		91,182,000
LHFS contractual principal outstanding	129,595,000		88,106,000
Fair value less unpaid principal	4,159,000		3,076,000
Level 2 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	335,244,000		317,858,000
Securities held to maturity	1,184,554,000		1,170,685,000
Deposits	9,906,990,000		9,698,358,000
Short-term liabilities	573,757,000		868,620,000
Long-term FHLB advances	1,222,000		1,253,000
Subordinated notes	49,944,000		49,936,000
Junior subordinated debt securities	61,856,000		61,856,000
Level 2 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	335,244,000		317,858,000
Securities held to maturity	1,212,505,000		1,182,846,000
Deposits	9,911,477,000		9,702,864,000
Short-term liabilities	573,757,000		868,620,000
Long-term FHLB advances	1,231,000		1,263,000
Subordinated notes	53,063,000		53,504,000
Junior subordinated debt securities	46,392,000		46,392,000
Level 3 [Member] | Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	6,342,555,000		6,379,853,000
Net acquired loans	486,606,000		537,350,000
FDIC indemnification asset	4,743,000		6,997,000
Level 3 [Member] | Estimate Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net LHFI	6,411,826,000		6,453,618,000
Net acquired loans	486,606,000		537,350,000
FDIC indemnification asset	$ 4,743,000		$ 6,997,000