Short-Term Borrowings and Long-Term Debt - Outstanding Short Term Borrowings and Long Term Debt (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
5.375% Senior Notes
Debt Outstanding [Line Items]
Stated interest rate	5.375%	5.375%
6.05% Subordinated Notes
Debt Outstanding [Line Items]
Stated interest rate	6.05%	6.05%
Fair value of the interest rate swap associated with the notes	8,500	9,000
6.05% Subordinated Notes \| Interest rate swaps
Debt Outstanding [Line Items]
Fair value of the interest rate swap associated with the notes	8,500	9,000
7.0% Junior Subordinated Debentures
Debt Outstanding [Line Items]
Stated interest rate	7.00%	7.00%

Fair value of the assets less the liabilities of a derivative or group of derivatives.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

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-Name Accounting Standards Codification

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