Investment Securities (Details) (USD $)	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Realized Gains (Losses), Sale Proceeds	$ 94,865,000	$ 0
Securities available for sale
Available-for-sale Securities, Amortized Cost	3,406,355,000	3,308,810,000		3,270,899,000
Available-for-sale Securities, Gross Unrealized Gains	102,037,000	71,899,000		100,416,000
Available-for-sale Securities, Gross Unrealized Losses	(16,745,000)	(17,958,000)		(17,762,000)
Available-for-sale	3,491,647,000	3,362,751,000	3,353,553,000	3,353,553,000
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	100,840,000	65,923,000		82,764,000
Held-to-maturity Securities, Unrecognized Holding Gain	3,025,000	1,980,000		2,348,000
Held-to-maturity Securities, Unrecognized Holding Loss	0	0		0
Held-to-maturity Securities, Fair Value	103,865,000	67,903,000		85,112,000
U.S. government agency debentures [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	35,340,000	409,090,000		122,711,000
Available-for-sale Securities, Gross Unrealized Gains	115,000	549,000		358,000
Available-for-sale Securities, Gross Unrealized Losses	0	(248,000)		0
Available-for-sale	35,455,000	409,391,000		123,069,000
US States and Political Subdivisions Debt Securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	365,543,000	292,531,000		334,916,000
Available-for-sale Securities, Gross Unrealized Gains	21,733,000	5,468,000		22,865,000
Available-for-sale Securities, Gross Unrealized Losses	(751,000)	(1,271,000)		(50,000)
Available-for-sale	386,525,000	296,728,000		357,731,000
Securities held to maturity
Held-to-maturity Securities, Amortized Cost	100,840,000	65,923,000		82,764,000
Held-to-maturity Securities, Unrecognized Holding Gain	3,025,000	1,980,000		2,348,000
Held-to-maturity Securities, Unrecognized Holding Loss	0	0		0
Held-to-maturity Securities, Fair Value	103,865,000	67,903,000		85,112,000
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	1,401,241,000	1,464,666,000		1,407,345,000
Available-for-sale Securities, Gross Unrealized Gains	53,388,000	49,591,000		53,129,000
Available-for-sale Securities, Gross Unrealized Losses	(213,000)	(1,737,000)		(131,000)
Available-for-sale	1,454,416,000	1,512,520,000		1,460,343,000
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	27,689,000
Available-for-sale Securities, Gross Unrealized Gains	34,000
Available-for-sale Securities, Gross Unrealized Losses	(60,000)
Available-for-sale	27,663,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	1,226,260,000	999,719,000		1,115,832,000
Available-for-sale Securities, Gross Unrealized Gains	25,046,000	16,194,000		22,058,000
Available-for-sale Securities, Gross Unrealized Losses	(2,000)	(2,557,000)		(55,000)
Available-for-sale	1,251,304,000	1,013,356,000		1,137,835,000
Non-agency [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	41,741,000	77,517,000		43,225,000
Available-for-sale Securities, Gross Unrealized Gains	91,000	97,000		82,000
Available-for-sale Securities, Gross Unrealized Losses	0	(5,000)		0
Available-for-sale	41,832,000	77,609,000		43,307,000
Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	147,214,000			127,624,000
Available-for-sale Securities, Gross Unrealized Gains	675,000			1,648,000
Available-for-sale Securities, Gross Unrealized Losses	(534,000)			(145,000)
Available-for-sale	147,355,000			129,127,000
Corporate Debt Securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	158,019,000	61,448,000		115,947,000
Available-for-sale Securities, Gross Unrealized Gains	955,000	0		276,000
Available-for-sale Securities, Gross Unrealized Losses	(15,185,000)	(12,140,000)		(17,381,000)
Available-for-sale	143,789,000	49,308,000		98,842,000
Total debt securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	3,403,047,000	3,304,971,000		3,267,600,000
Available-for-sale Securities, Gross Unrealized Gains	102,037,000	71,899,000		100,416,000
Available-for-sale Securities, Gross Unrealized Losses	(16,745,000)	(17,958,000)		(17,762,000)
Available-for-sale	3,488,339,000	3,358,912,000		3,350,254,000
Marketable equity securities [Member]
Securities available for sale
Available-for-sale Securities, Amortized Cost	3,308,000	3,839,000		3,299,000
Available-for-sale Securities, Gross Unrealized Gains	0	0		0
Available-for-sale Securities, Gross Unrealized Losses	0	0		0
Available-for-sale	$ 3,308,000	$ 3,839,000		$ 3,299,000

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15E

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 21

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossRealizedGainsLossesSaleProceeds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13, 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesGrossUnrealizedGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesGrossUnrealizedLosses1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents the amortized cost, prior to other than temporary impairment (OTTI), of investments in debt securities which are categorized as held-to-maturity. The held-to-maturity category is for those securities that the entity has the positive intent and ability to hold until maturity. The temporary impairments arise from the unrealized losses that are recognized in AOCI.

+ References

No definition available.

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt securities as of the balance sheet date and which are categorized as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_HeldToMaturitySecuritiesUnclassifiedAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

As of the balance sheet date, the excess of fair value over carrying value (amortized cost) of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding gain.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldtomaturitySecuritiesUnrecognizedHoldingGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

As of the balance sheet date, the excess of carrying value (amortized cost) over fair value of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 137

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27232-111563

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesUnrecognizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration