Securities (Details 3) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2013 item	Jun. 30, 2013 item	Jun. 30, 2012	Dec. 31, 2012 item
Securities
Number of securities held with unrealized loss	76	76		41
Proceeds from sales of securities available-for-sale		$ 41,640,000	$ 54,318,000
Gross realized gains from sale of available for sale securities	2,057,000	2,057,000	2,477,000
Total available-for-sale securities	342,591,000	342,591,000		336,300,000
Held-to-maturity, amortized cost	1,892,883,000	1,892,883,000		1,700,141,000
Held-to-maturity, Estimated fair value	1,882,671,000	1,882,671,000		1,725,090,000
Accrued interest receivable for securities	6,200,000	6,200,000		5,700,000
Callable securities, amortized cost	179,600,000	179,600,000
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity, amortized cost	81,500,000	81,500,000
Held-to-maturity, Estimated fair value	77,500,000	77,500,000
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	98,700,000	98,700,000
Total available-for-sale securities	$ 93,500,000	$ 93,500,000