Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Available-for-sale Securities
Securities available for sale, Fair Value	$ 4,496,900	$ 1,488,885
Short-term investments	0	274,974
Total assets	4,511,852	1,766,811
Liabilities
Total Liabilities	15,643	2,952
U.S. Treasury and government agency securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	250,067	117,435
Debt securities issued by states of the United States and political subdivisions of the states [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	309,665	180,443
Corporate debt securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	4,494	15,285
Residential mortgage-backed securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	2,480,345	799,686
Collateralized mortgage obligations [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	1,446,076	372,051
Equity securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	6,253	3,985
Derivative financial instruments-assets [Member]
Available-for-sale Securities
Derivative financial instruments - assets	14,952	2,952
Derivative financial instruments-liabilities [Member]
Liabilities
Derivative financial instruments - liabilities	15,643	2,952
Level 1 [Member]
Available-for-sale Securities
Short-term investments		274,974
Total assets	260,814	411,679
Liabilities
Total Liabilities	0	0
Level 1 [Member] \| U.S. Treasury and government agency securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	250,067	117,435
Level 1 [Member] \| Debt securities issued by states of the United States and political subdivisions of the states [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	0	0
Level 1 [Member] \| Corporate debt securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	4,494	15,285
Level 1 [Member] \| Residential mortgage-backed securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	0	0
Level 1 [Member] \| Collateralized mortgage obligations [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	0	0
Level 1 [Member] \| Equity securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	6,253	3,985
Level 1 [Member] \| Derivative financial instruments-assets [Member]
Available-for-sale Securities
Derivative financial instruments - assets	0	0
Level 1 [Member] \| Derivative financial instruments-liabilities [Member]
Liabilities
Derivative financial instruments - liabilities	0	0
Level 2 [Member]
Available-for-sale Securities
Short-term investments		0
Total assets	4,251,038	1,355,132
Liabilities
Total Liabilities	15,643	2,952
Level 2 [Member] \| U.S. Treasury and government agency securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	0	0
Level 2 [Member] \| Debt securities issued by states of the United States and political subdivisions of the states [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	309,665	180,443
Level 2 [Member] \| Corporate debt securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	0	0
Level 2 [Member] \| Residential mortgage-backed securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	2,480,345	799,686
Level 2 [Member] \| Collateralized mortgage obligations [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	1,446,076	372,051
Level 2 [Member] \| Equity securities [Member]
Available-for-sale Securities
Securities available for sale, Fair Value	0	0
Level 2 [Member] \| Derivative financial instruments-assets [Member]
Available-for-sale Securities
Derivative financial instruments - assets	14,952	2,952
Level 2 [Member] \| Derivative financial instruments-liabilities [Member]
Liabilities
Derivative financial instruments - liabilities	$ 15,643	$ 2,952

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

- Definition

Fair values as of the balance sheet date of all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments, net of the effects of master netting arrangements.

+ References+ Details

- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

+ References+ Details

- Definition

Other investments not otherwise specified in the taxonomy that will be realized in a short period of time, usually less than one year or the normal operating cycle, whichever is longer.

+ References+ Details