Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other Income And Expenses [Abstract]
Change in COLI policies	$ 2,500	$ 2,200	$ 8,100	$ 5,500	$ 9,200	$ 8,100
Interest income	158	280	422	785	561	962
Pipe replacement costs	(42)	(973)	(163)	(2,015)	(828)	(3,663)
Miscellaneous income and (expense)	52	129	(170)	304	(907)	577
Total other income (deductions)	$ 2,668	$ 1,636	$ 8,189	$ 4,574	$ 8,026	$ 5,976