Summary of Estimated Fair Values of Financial Instruments that are not Carried at Fair Value (Parenthetical) (Detail) (Interest rate swaps, USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
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	$ 6,100	$ 15,700
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	7,200	36,300
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	6,100	15,700
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	7,200	36,300
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	6,100	15,700
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	$ 7,200	$ 36,300