Share-Based Compensation - Equity-settled share-base payment plan (Detail) (T Bucks Employee Participation Plan [Member], USD $)	9 Months Ended
	Sep. 30, 2012
T Bucks Employee Participation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Balance at December 31, 2011	548,234
Shares acquired by the Trust	548,234
Balance at September 30, 2012	548,234
Outstanding awards expected to vest	548,234
Balance at December 31, 2011	$ 25.79	[1]
Shares acquired by the Trust	$ 25.79	[1]
Balance at September 30, 2012	$ 25.79	[1]
Outstanding awards expected to vest	$ 25.79	[1]

[1] Represents the fair value on the date of purchase by the Trust.