Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Parenthetical) (Detail) (Interest rate swaps, USD $) In Thousands, unless otherwise specified	Jun. 30, 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	9,300	8,800
Carrying Amount | 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
Carrying Amount | 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	9,300	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	$ 9,300	$ 8,800