Investment Securities (Details 2) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Held-to-maturity debt securities amortized cost disclosures
Due in one year or less, held-to-maturity debt securities amortized cost	$ 25,000		$ 25,000
Due after one year through five years, held-to-maturity debt securities amortized cost	2,400,000		2,400,000
Amortized cost, held-to-maturity debt securities	2,425,000		2,425,000
Held-to-maturity debt securities, Estimated fair value disclosures
Due in one year or less, held-to-maturity debt securities, Estimated fair value	25,000		25,000
Due after one year through five years, held-to-maturity debt securities, Estimated fair value	2,400,000		2,400,000
Estimated fair value, held-to-maturity debt securities	2,425,000		2,425,000
Available-for-sale debt securities amortized cost disclosures
Due after five years through ten years, available-for-sale debt securities amortized cost	1,422,000		1,422,000
Due after ten years, available-for-sale debt securities amortized cost	215,278,000		215,278,000
Residential mortgage-backed securities, amortized cost	4,841,592,000		4,841,592,000
Equity securities, amortized cost	19,575,000		19,575,000
Amortized cost, Available for sale securities	5,077,867,000		5,077,867,000		5,082,095,000
Available for sale debt securities, Estimated Fair Value Disclosures
Due after five years through ten years, available-for-sale debt securities, Estimated Fair Value	1,518,000		1,518,000
Due after ten years, available-for-sale debt securities, Estimated Fair Value	232,381,000		232,381,000
Residential mortgage-backed securities, Estimated Fair Value	4,953,594,000		4,953,594,000
Equity securities, Estimated Fair Value	21,110,000		21,110,000
Estimated fair value, Available for sale securities	5,208,603,000		5,208,603,000		5,213,915,000
Amortized cost of available for sale investment securities pledged	2,653,307,000		2,653,307,000
Fair value of available for sale investment securities pledged	2,742,643,000		2,742,643,000
Proceeds from sales and calls of available for sale securities	55,052,000	422,177,000	72,383,000	774,856,000
Proceeds from sales of mortgage-backed securities	31,515,000	420,149,000	31,515,000	770,244,000
Gross gains realized on sales	1,420,000	1,458,000	2,593,000	2,893,000
Gross losses realized on sales		(13,000)	(1,000)	(32,000)
Impairment charges on available-for-sale investment securities	222,000	166,000	408,000	415,000
Impairment charges on available-for-sale investment securities, after tax	$ 144,300	$ 107,900	$ 265,200	$ 269,750