Derivative Financial Instruments - Additional Information (Detail) (USD $) In Millions, except Per Share data	1 Months Ended	9 Months Ended
	Apr. 30, 2008	Sep. 30, 2011
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Managing interest rate risk for senior notes and subordinated notes		To manage interest rate risk for our 5.70% Senior Notes, and 6.05% Subordinated Notes, we entered into fixed-for-floating interest rate swap agreements at the time of debt issuance based upon London Interbank Offered Rates ("LIBOR") with matched-terms.
3.875% Convertible Notes
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Convertible notes initial conversion price before hedge and warrant agreement	$ 53.04
3.875% Convertible Notes | Hedge and warrant agreement
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Convertible note hedge and warrant agreement cost	$ 20.6
Price per share of the conversion feature for the hedge and warrant agreement associated with our convertible notes	$ 64.43	$ 64.43
Period of warrants ratable expiration		60 business days beginning on July 15, 2011.
Begin date of warrants ratable expiration		2011-07-15