Summary of Held-To-Maturity and Available-For-Sale Investment Securities (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Investment Holdings [Line Items]
Held-to-Maturity Amortized Cost	$ 3,001	$ 17,406
Held-to-Maturity Unrealized Gains	225	678
Held-to-Maturity Unrealized Losses	0	(18)
Held-to-Maturity Market Value	3,226	18,066
Available-for-Sale Amortized Cost	1,114,869	904,546
Available-for-Sale Unrealized Gains	20,074	17,291
Available-for-Sale Unrealized Losses	(829)	(2,727)
Available-for-Sale Market Value	1,134,114	919,110
U.S. Treasuries
Investment Holdings [Line Items]
Held-to-Maturity Amortized Cost		13,959
Held-to-Maturity Unrealized Gains		390
Held-to-Maturity Unrealized Losses		(18)
Held-to-Maturity Market Value		14,331
Available-for-Sale Amortized Cost		0
Available-for-Sale Unrealized Gains		0
Available-for-Sale Unrealized Losses		0
Available-for-Sale Market Value		0
Securities of U.S. government agencies and corporations
Investment Holdings [Line Items]
Held-to-Maturity Amortized Cost	0	0
Held-to-Maturity Unrealized Gains	0	0
Held-to-Maturity Unrealized Losses	0	0
Held-to-Maturity Market Value	0	0
Available-for-Sale Amortized Cost	5,027	105,028
Available-for-Sale Unrealized Gains	149	957
Available-for-Sale Unrealized Losses	0	0
Available-for-Sale Market Value	5,176	105,985
Mortgage-backed securities
Investment Holdings [Line Items]
Held-to-Maturity Amortized Cost	103	118
Held-to-Maturity Unrealized Gains	2	4
Held-to-Maturity Unrealized Losses	0	0
Held-to-Maturity Market Value	105	122
Available-for-Sale Amortized Cost	1,087,462	775,867
Available-for-Sale Unrealized Gains	19,311	15,513
Available-for-Sale Unrealized Losses	(776)	(2,630)
Available-for-Sale Market Value	1,105,997	788,750
Obligations of state and other political subdivisions
Investment Holdings [Line Items]
Held-to-Maturity Amortized Cost	2,898	3,329
Held-to-Maturity Unrealized Gains	223	284
Held-to-Maturity Unrealized Losses	0	0
Held-to-Maturity Market Value	3,121	3,613
Available-for-Sale Amortized Cost	12,187	13,708
Available-for-Sale Unrealized Gains	173	207
Available-for-Sale Unrealized Losses	(52)	(91)
Available-for-Sale Market Value	12,308	13,824
Other securities
Investment Holdings [Line Items]
Held-to-Maturity Amortized Cost	0	0
Held-to-Maturity Unrealized Gains	0	0
Held-to-Maturity Unrealized Losses	0	0
Held-to-Maturity Market Value	0	0
Available-for-Sale Amortized Cost	10,193	9,943
Available-for-Sale Unrealized Gains	441	614
Available-for-Sale Unrealized Losses	(1)	(6)
Available-for-Sale Market Value	$ 10,633	$ 10,551

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

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

- 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+ Details

- 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+ Details

- Definition

For an unclassified balance sheet, this item represents 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Accounting Research Bulletin (ARB)
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+ Details

- 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+ Details

- 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+ Details

- 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+ Details