Investment Securities (Details 2) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Amortized Cost, Available-for-Sale
Available-for-Sale, Due in One Year or Less	$ 288,391,000
Available-for-Sale, Due After One Year Through Five Years	194,296,000
Available-for-Sale, Due After Five Years Through Ten Years	643,116,000
Available-for-Sale, Due After Ten Years	570,432,000
Available-for-Sale, Total	1,696,235,000
Fair Value, Available-for-Sale
Available-for-Sale, Due in One Year or Less	289,275,000
Available-for-Sale, Due After One Year Through Five Years	201,181,000
Available-for-Sale, Due After Five Years Through Ten Years	653,907,000
Available-for-Sale, Due After Ten Years	591,788,000
Available-for-Sale, Total	1,736,151,000
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held-to-Maturity, Due in One Year or Less	60,160,000
Held-to-Maturity, Due After One Year Through Five Years	224,979,000
Held-to-Maturity, Due After Ten Years	23,544,000
Held-to-Maturity, Total	308,683,000
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held-to-Maturity, Due in One Year or Less	60,543,000
Held-to-Maturity, Due After One Year Through Five Years	229,803,000
Held-to-Maturity, Due After Ten Years	23,372,000
Held-to-Maturity, Total	313,718,000
Available-for-Sale:
Amortized Cost	3,227,554,000		3,291,685,000
Fair Value	3,290,850,000		3,367,557,000
Held-to-Maturity
Amortized Cost	3,597,810,000		3,595,065,000
Fair Value	3,679,208,000		3,687,676,000
Carrying value of investment securities which are pledged	2,500,000,000		2,900,000,000
Proceeds from Sale and Maturity of Marketable Securities	0
Gross gains and losses from sale of investment securities
Gross realized gains on sale of investment securities		200,000
Gross losses on sale of investment securities		300,000
Residential Mortgage-Backed Securities issued by U.S. government agencies
Available-for-Sale:
Amortized Cost	919,419,000		1,041,669,000
Fair Value	944,362,000		1,068,660,000
Held-to-Maturity
Amortized Cost	3,260,767,000		3,349,403,000
Fair Value	3,335,290,000		3,434,710,000
Residential Mortgage-Backed Securities Issued by U.S. Government Sponsored Enterprises
Available-for-Sale:
Amortized Cost	31,538,000		35,234,000
Fair Value	33,398,000		37,298,000
Held-to-Maturity
Amortized Cost	28,360,000		31,494,000
Fair Value	30,200,000		33,596,000
Commercial Mortgage-Backed Securities Issued by Government Agencies
Available-for-Sale:
Amortized Cost	580,362,000		524,055,000
Fair Value	576,939,000		524,765,000
Mortgage-Backed Securities
Available-for-Sale:
Amortized Cost	1,531,319,000		1,600,958,000
Fair Value	1,554,699,000		1,630,723,000
Held-to-Maturity
Amortized Cost	3,289,127,000		3,380,897,000
Fair Value	$ 3,365,490,000		$ 3,468,306,000