Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Other Income And Expenses [Abstract]
Change in COLI policies	$ 1,800	$ (1,900)	$ 5,600	$ 3,300	$ 8,900	$ (800)
Interest income	144	279	264	505	683	800
Pipe replacement costs	(36)	(1,033)	(121)	(1,042)	(1,759)	(3,956)
Miscellaneous income and (expense)	(456)	164	(222)	175	(830)	209
Total other income (deductions)	$ 1,452	$ (2,490)	$ 5,521	$ 2,938	$ 6,994	$ (3,747)