Investment Securities (Details 4) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012
Securities Available for Sale
One year or less	$ 100,074
Over one year through five years	2,616,784
Over five years through ten years	559,564
Over ten years	211,917
Fair Value	3,488,339
Weighted Average Yield, one year or less	2.98%
Weighted Average Yield, over one year through five years	2.75%
Weighted Average Yield, over five years through ten years	3.34%
Weighted Average Yield, over ten years	41.60%
Weighted Average Yield, Total	2.94%
Amortized Cost	3,403,047
Weighted-Average Yield	2.94%
Weighted-Average Maturity	4.58
Securities Held to Maturity
One year or less	40,713
Over one year through five years	15,098
Over five years through ten years	5,204
Over ten years	42,850
Fair Value	103,865
Weighted Average Yield, one year or less	1.53%
Weighted Average Yield, over one year through five years	4.52%
Weighted Average Yield, over five years through ten years	5.46%
Weighted Average Yield, over ten years	7.16%
Weighted Average Yield, Total	4.49%
Amortized Cost	100,840
Weighted Average Yield	4.49%
Weighted-Average Maturity	6.4200
U.S. government agency debentures [Member]
Securities Available for Sale
One year or less	35,455
Over one year through five years	0
Over five years through ten years	0
Over ten years	0
Fair Value	35,455
Amortized Cost	35,340
Weighted-Average Yield	0.73%
Weighted-Average Maturity	0.65
US States and Political Subdivisions Debt Securities [Member]
Securities Available for Sale
One year or less	8,808
Over one year through five years	25,781
Over five years through ten years	186,676
Over ten years	165,260
Fair Value	386,525
Amortized Cost	365,543
Weighted-Average Yield	5.39%
Weighted-Average Maturity	10.21
Securities Held to Maturity
One year or less	40,713
Over one year through five years	15,098
Over five years through ten years	5,204
Over ten years	42,850
Fair Value	103,865
Amortized Cost	100,840
Weighted Average Yield	4.49%
Weighted-Average Maturity	6.42
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Available for Sale
One year or less	4,408
Over one year through five years	1,291,288
Over five years through ten years	158,720
Over ten years	0
Fair Value	1,454,416
Amortized Cost	1,401,241
Weighted-Average Yield	3.10%
Weighted-Average Maturity	3.60
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	5,067
Over five years through ten years	22,596
Over ten years	0
Fair Value	27,663
Amortized Cost	27,689
Weighted-Average Yield	0.59%
Weighted-Average Maturity	5.98
Residential Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities Available for Sale
One year or less	51,403
Over one year through five years	1,174,567
Over five years through ten years	25,334
Over ten years	0
Fair Value	1,251,304
Amortized Cost	1,226,260
Weighted-Average Yield	2.43%
Weighted-Average Maturity	3.36
Non-agency [Member]
Securities Available for Sale
One year or less	0
Over one year through five years	41,832
Over five years through ten years	0
Over ten years	0
Fair Value	41,832
Amortized Cost	41,741
Weighted-Average Yield	0.52%
Weighted-Average Maturity	2.35
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	54,273
Over five years through ten years	93,082
Over ten years	0
Fair Value	147,355
Amortized Cost	147,214
Weighted-Average Yield	2.47%
Weighted-Average Maturity	6.02
Corporate Debt Securities [Member]
Securities Available for Sale
One year or less	0
Over one year through five years	23,976
Over five years through ten years	73,156
Over ten years	46,657
Fair Value	143,789
Amortized Cost	$ 158,019
Weighted-Average Yield	1.85%
Weighted-Average Maturity	9.60