Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of the interest rate swap associated with the notes		$ 2,600
6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of the interest rate swap associated with the notes	9,000	8,800
Estimated Fair Value | 5.70% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of the interest rate swap associated with the notes		2,600
Estimated Fair Value | 6.05% Subordinated Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of the interest rate swap associated with the notes	$ 9,000	$ 8,800