Fair Value Measurements - Additional Information (Detail) (Level 1 inputs, Senior Unsecured Notes, USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Level 1 inputs | Senior Unsecured Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Unsecured Notes, fair Value	$ 1,319.6	$ 1,328.1