Derivative Financial Instruments, Average Remaining Maturity and the Weighted-Average Interest Rates being Paid and Received (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Derivative [Line Items]
Notional Value	$ 703,286	$ 578,959
Average Maturity (years)	P4.4Y
Fair Value	(2,871)
Weighted-Average Rate Receive	4.34%
Weighted-Average Rate Pay	4.47%
Pay fixed interest rate swaps with counterparty
Derivative [Line Items]
Notional Value	20,310
Average Maturity (years)	P4.4Y
Fair Value	(2,108)
Weighted-Average Rate Receive	2.23%
Weighted-Average Rate Pay	6.79%
Matched interest rate swaps \| Receive fixed
Derivative [Line Items]
Notional Value	341,488
Average Maturity (years)	P4.4Y
Fair Value	16,729
Weighted-Average Rate Receive	5.87%
Weighted-Average Rate Pay	2.93%
Matched interest rate swaps \| Pay fixed
Derivative [Line Items]
Notional Value	341,488
Average Maturity (years)	P4.4Y
Fair Value	(17,492)
Weighted-Average Rate Receive	2.93%
Weighted-Average Rate Pay	5.87%
Asset conversion swaps
Derivative [Line Items]
Notional Value	703,286
Average Maturity (years)	P4.4Y
Fair Value	$ (2,871)
Weighted-Average Rate Receive	4.34%
Weighted-Average Rate Pay	4.47%

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 107
-Paragraph 10
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 133
-Paragraph 4, 17
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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- Definition

Average fixed interest rate related to the group of interest rate derivatives.

+ Details

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- Definition

The average time remaining until maturity on the group of derivatives identified.

+ Details

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- Definition

Average variable interest rate related to the group of interest rate derivatives.

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Data Type:	num:percentItemType
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

Name:	us-gaap_DerivativeLineItems
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- Definition

Aggregate notional amount of all derivatives. The notional amount relates to a number of currency units, shares, bushels, pounds, or other units specified in a derivative instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Notional Amount
-URI http://asc.fasb.org/extlink&oid=6519104

+ Details

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