Securities (Details 3) (USD $)	3 Months Ended
	Mar. 31, 2013 item	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2012 item
Securities
Number of securities held with unrealized loss	42			41
Proceeds from sales of securities available-for-sale	$ 0	$ 54,318,000	$ 0
Gross realized gains from sale of available for sale securities		2,477,000
Total available-for-sale securities	428,374,000			336,300,000
Held-to-maturity, amortized cost	1,709,427,000			1,700,141,000
Held-to-maturity, Estimated fair value	1,732,726,000			1,725,090,000
Accrued interest receivable for securities	6,100,000			5,700,000
Callable securities, amortized cost	179,600,000
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity, amortized cost	81,600,000
Held-to-maturity, Estimated fair value	81,600,000
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	98,700,000
Total available-for-sale securities	$ 98,600,000