Fair Values of Financial Instruments - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Series A, Senior Notes, 4.97 Percent, Due October 5th 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Current market rates used in measuring fair value	4.26%	3.36%
Series B, Senior Notes, 4.27 Percent, Due June 14th 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Current market rates used in measuring fair value	4.61%	3.70%