Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (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
Equity [Abstract]
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 334	$ 668	$ 667	$ 1,336	$ 845
Amortization of net actuarial (gain)/loss, Before-Tax Amount	6,766	8,595	13,533	17,191	30,723	29,019
Regulatory adjustment, Before-Tax Amount	(6,123)	(7,787)	(12,246)	(15,574)	(2,318)	135,089
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	977	1,142	1,955	2,284	(778)	(15,354)
Amounts reclassified into net income, Before-Tax Amount	837	836	1,673	1,672	3,345	3,344
FSIRS other comprehensive income (loss), Before-Tax Amount	837	836	1,673	1,672	3,345	3,344
Translation adjustments, Before-Tax Amount	70	209	852	(1,063)	(39)	(1,722)
Foreign currency other comprehensive income (loss), Before-Tax Amount	70	209	852	(1,063)	(39)	(1,722)
Total other comprehensive income (loss), Before-Tax Amount	1,884	2,187	4,480	2,893	2,528	(13,732)
Amortization of prior service cost, Tax (Expense) or Benefit	(127)	(126)	(254)	(253)	(508)	(321)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(2,572)	(3,267)	(5,143)	(6,533)	(11,675)	(11,027)
Regulatory adjustment, Tax (Expense) or Benefit	2,327	2,959	4,654	5,918	882	(51,334)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(372)	(434)	(743)	(868)	296	5,834
Amounts reclassified into net income, Tax (Expense) or Benefit	(318)	(318)	(635)	(635)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(318)	(318)	(635)	(635)	(1,271)	(1,271)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	(690)	(752)	(1,378)	(1,503)	(975)	4,563
Amortization of prior service cost, Net-of-Tax Amount	207	208	414	414	828	524
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	4,194	5,328	8,390	10,658	19,048	17,992
Regulatory adjustment, Net-of-Tax Amount	(3,796)	(4,828)	(7,592)	(9,656)	(1,436)	83,755
Pension plans other comprehensive income (loss), Net-of-Tax Amount	605	708	1,212	1,416	(482)	(9,520)
Amounts reclassified into net income	519	518	1,038	1,037	2,074	2,073
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	518	1,038	1,037	2,074	2,073
Translation adjustments, Net-of-Tax Amount	70	209	852	(1,063)	(39)	(1,722)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	70	209	852	(1,063)	(39)	(1,722)
Total other comprehensive income (loss), net of tax	1,194	1,435	3,102	1,390	1,553	(9,169)
Net actuarial gain/(loss), Before-Tax Amount					(30,519)	(173,646)
Prior service cost, Before-Tax Amount					0	(6,661)
Net actuarial gain/(loss), Tax (Expense) or Benefit					11,597	65,985
Prior service cost, Tax (Expense) or Benefit					0	2,531
Net actuarial gain/(loss), Net-of-Tax Amount	0	0	0	0	(18,922)	(107,661)
Prior service cost, Net-of-Tax Amount	$ 0	$ 0	$ 0	$ 0	$ 0	$ (4,130)