Nature of Operations and Basis of Presentation - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013 Plant materials and operating supplies [Member]	Dec. 31, 2012 Plant materials and operating supplies [Member]
Plant materials and operating supplies		$ 25	$ 25
Noncash construction advances - noncash investing item	$ 4.8