Nature of Operations and Basis of Presentation - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Other Income and Expenses [Abstract]
Change in COLI policies	$ 2,200	$ 0	$ 3,100	$ 1,300	$ 1,300	$ 3,400
Interest income	391	161	758	751	2,180	2,244
Equity AFUDC	750	737	1,282	1,133	3,157	2,104
Foreign transaction gain (loss)	(9)	(245)	(19)	(572)	(271)	(750)
Miscellaneous income and (expense)	(862)	(491)	(930)	(178)	(1,730)	(1,932)
Total other income (deductions)	$ 2,470	$ 162	$ 4,191	$ 2,434	$ 4,636	$ 5,066