Investment Securities (Details 4) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Securities Available for Sale
One year or less	$ 114,333
Over one year through five years	2,467,708
Over five years through ten years	233,188
Over ten years	93,519
Fair Value	2,908,748
Weighted Average Yield, one year or less	2.68%
Weighted Average Yield, over one year through five years	2.55%
Weighted Average Yield, over five years through ten years	4.74%
Weighted Average Yield, over ten years	2.74%
Weighted Average Yield, Total	2.74%
Amortized Cost	2,824,577
Weighted-Average Yield	2.74%
Weighted-Average Maturity	3.66
Securities Held to Maturity
One year or less	45,126
Over one year through five years	199,466
Over five years through ten years	208,176
Over ten years	178,469
Fair Value	631,237
Weighted Average Yield, one year or less	1.61%
Weighted Average Yield, over one year through five years	2.17%
Weighted Average Yield, over five years through ten years	2.81%
Weighted Average Yield, over ten years	5.14%
Weighted Average Yield, Total	3.18%
Amortized Cost	620,631
Weighted Average Yield	3.18%
Weighted-Average Maturity	7.35
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Held to Maturity
Amortized Cost	33,149
Weighted-Average Maturity	6.03
US Government Agencies Debt Securities [Member]
Securities Available for Sale
One year or less	35,137
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Fair Value	35,137
Amortized Cost	35,109
Weighted-Average Yield	0.73%
Weighted-Average Maturity	0.14
US States and Political Subdivisions Debt Securities [Member]
Securities Available for Sale
One year or less	7,322
Over one year through five years	27,234
Over five years through ten years	177,836
Over ten years	46,026
Fair Value	258,418
Amortized Cost	241,436
Weighted-Average Yield	5.37%
Weighted-Average Maturity	7.83
Securities Held to Maturity
Amortized Cost	262,091
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Available for Sale
One year or less	4,032
Over one year through five years	1,265,886
Over five years through ten years	7,913
Over ten years	0
Fair Value	1,277,831
Amortized Cost	1,216,671
Weighted-Average Yield	2.98%
Weighted-Average Maturity	3.02
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities Available for Sale
One year or less	5,073
Over one year through five years	0
Over five years through ten years	10,594
Over ten years	0
Fair Value	15,667
Amortized Cost	15,434
Weighted-Average Yield	1.75%
Weighted-Average Maturity	4.00
Residential Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Available for Sale
One year or less	62,769
Over one year through five years	1,111,218
Over five years through ten years	31,538
Over ten years	0
Fair Value	1,205,525
Amortized Cost	1,188,072
Weighted-Average Yield	2.14%
Weighted-Average Maturity	2.94
Securities Held to Maturity
Amortized Cost	129,265
Weighted-Average Maturity	2.7
Non-agency [Member]
Securities Available for Sale
One year or less	0
Over one year through five years	12
Over five years through ten years	0
Over ten years	0
Fair Value	12
Amortized Cost	12
Weighted-Average Yield	3.81%
Weighted-Average Maturity	1.92
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Available for Sale
One year or less	0
Over one year through five years	63,358
Over five years through ten years	5,307
Over ten years	0
Fair Value	68,665
Amortized Cost	66,316
Weighted-Average Yield	2.13%
Weighted-Average Maturity	4.35
Securities Held to Maturity
Amortized Cost	98,961
Weighted-Average Maturity	6.86
Corporate Debt Securities [Member]
Securities Available for Sale
One year or less	0
Over one year through five years	0
Over five years through ten years	0
Over ten years	47,493
Fair Value	47,493
Amortized Cost	61,527
Weighted-Average Yield	1.09%
Weighted-Average Maturity	15.06
Securities Held to Maturity
Amortized Cost	97,165
Residential Mortgage Backed Securities [Member]
Securities Held to Maturity
Fair Value	0
Corporate Debt Securities [Member]
Securities Held to Maturity
Over one year through five years	33,529
Over five years through ten years	65,071
Fair Value	98,600
Weighted Average Yield	2.23%
Weighted-Average Maturity	5.29
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Held to Maturity
Over one year through five years	21,160
Over five years through ten years	78,855
Fair Value	100,015
Weighted Average Yield	2.91%
Residential Collateralized Mortgage Backed Securities Issued by Non Agency [Member]
Securities Held to Maturity
Fair Value	0
Residential Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Held to Maturity
Over one year through five years	129,661
Fair Value	129,661
Weighted Average Yield	1.89%
US States and Political Subdivisions Debt Securities [Member]
Securities Held to Maturity
One year or less	45,126
Over one year through five years	15,116
Over five years through ten years	30,309
Over ten years	178,469
Fair Value	269,020
Weighted Average Yield	5.10%
Weighted-Average Maturity	13.48
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Held to Maturity
Over five years through ten years	33,941
Fair Value	$ 33,941
Weighted Average Yield	1.42%