Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Parenthetical) (Detail) (6.05% Subordinated Notes, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
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	$ 8,500	$ 9,000