Securities (Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities) (Details) (USD $) In Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Held to Maturity, Amortized Cost	$ 349,997	$ 283,629	$ 252,995
Held to Maturity, Gross Unrealized Gains	21,323	13,653
Held to Maturity, Gross Unrealized Losses	345	1,054
Held to Maturity, Estimated Fair Value	370,975	296,228
Available for Sale, Amortized Cost	5,193,684	5,009,759
Available for Sale, Gross Unrealized Gains	241,226	170,374
Available for Sale, Gross Unrealized Losses	2,556	22,663
Available for Sale, Estimated Fair Value	5,432,354	5,157,470	4,753,608
U.S. Treasury [Member]
Held to Maturity, Amortized Cost	247,606	247,421
Held to Maturity, Gross Unrealized Gains	19,581	13,517
Held to Maturity, Gross Unrealized Losses
Held to Maturity, Estimated Fair Value	267,187	260,938
Available for Sale, Amortized Cost	773,274	973,033
Available for Sale, Gross Unrealized Gains	21,011	13,998
Available for Sale, Gross Unrealized Losses
Available for Sale, Estimated Fair Value	794,285	987,031
Residential Mortgage-Backed Securities [Member]
Held to Maturity, Amortized Cost	4,103	4,405
Held to Maturity, Gross Unrealized Gains	165	136
Held to Maturity, Gross Unrealized Losses
Held to Maturity, Estimated Fair Value	4,268	4,541
Available for Sale, Amortized Cost	2,356,226	1,989,299
Available for Sale, Gross Unrealized Gains	128,439	103,018
Available for Sale, Gross Unrealized Losses	2	987
Available for Sale, Estimated Fair Value	2,484,663	2,091,330
States and Political Subdivisions [Member]
Held to Maturity, Amortized Cost	97,288	30,803
Held to Maturity, Gross Unrealized Gains	1,577
Held to Maturity, Gross Unrealized Losses	345	1,054
Held to Maturity, Estimated Fair Value	98,520	29,749
Available for Sale, Amortized Cost	2,026,204	2,008,618
Available for Sale, Gross Unrealized Gains	91,776	53,358
Available for Sale, Gross Unrealized Losses	2,554	21,676
Available for Sale, Estimated Fair Value	2,115,426	2,040,300
Other [Member]
Held to Maturity, Amortized Cost	1,000	1,000
Held to Maturity, Gross Unrealized Gains
Held to Maturity, Gross Unrealized Losses
Held to Maturity, Estimated Fair Value	1,000	1,000
Available for Sale, Amortized Cost	37,980	38,809
Available for Sale, Gross Unrealized Gains
Available for Sale, Gross Unrealized Losses
Available for Sale, Estimated Fair Value	$ 37,980	$ 38,809

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading. Such securities are reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph b

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 14

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments made for accretion, amortization, other-than-temporary impairments, and hedging, 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 19

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- Definition

This item represents the gross unrealized gains for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized gains are the excess of the fair value of the Available-for-sale Securities over their carrying value as of the reporting date. Such gross unrealized gains are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge. All or a portion of the unrealized holding gain of an available-for-sale security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13, 19

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- Definition

This item represents the gross unrealized losses for securities which are categorized neither as held-to-maturity nor trading securities. Such gross unrealized losses are the excess of the carrying value of the Available-for-sale Securities over their fair value as of the reporting date. Such gross unrealized losses are included in other comprehensive income in the statement of shareholders' equity, unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

+ References

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- Definition

For an unclassified balance sheet, this item represents investments in debt securities which are categorized as held-to-maturity; such investments are measured at amortized cost (carrying value). The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
-Number 43
-Chapter 3
-Section A
-Paragraph 4, 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 17

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 7, 8, 9, 10, 11

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Balance Type:	debit
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- 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 137

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- 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

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- 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

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