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		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount					$ (173,646)	$ 100,345
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 89	$ 1,001	$ 266	1,090	355
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,595	5,914	25,786	17,742	31,700	26,285
Prior service cost, Before-Tax Amount					(6,661)	0
Regulatory adjustment, Before-Tax Amount	(7,787)	(5,178)	(23,361)	(15,533)	132,480	(116,412)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	3,426	2,475	(15,037)	10,573
Amounts reclassified into net income, Before-Tax Amount	836	836	2,508	2,509	3,344	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	2,508	2,509	3,344	3,345
Translation adjustments, Before-Tax Amount	(510)	0	(1,573)	0	(2,232)	0
Foreign currency other comprehensive income (loss), Before-Tax Amount	(510)	0	(1,573)	0	(2,232)	0
Total other comprehensive income (loss), Before-Tax Amount	1,468	1,661	4,361	4,984	(13,925)	13,918
Net actuarial gain/(loss), Tax (Expense) or Benefit					65,985	(38,131)
Amortization of prior service cost, Tax (Expense) or Benefit	(127)	(34)	(380)	(101)	(414)	(134)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(3,266)	(2,247)	(9,799)	(6,742)	(12,046)	(9,988)
Prior service cost, Tax (Expense) or Benefit					2,531	0
Regulatory adjustment, Tax (Expense) or Benefit	2,959	1,968	8,877	5,903	(50,343)	44,236
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(313)	(1,302)	(940)	5,713	(4,017)
Amounts reclassified into net income, Tax (Expense) or Benefit	(317)	(318)	(952)	(954)	(1,270)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(317)	(318)	(952)	(954)	(1,270)	(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	(751)	(631)	(2,254)	(1,894)	4,443	(5,288)
Net actuarial gain/(loss), Net-of-Tax Amount	0	0	0	0	(107,661)	62,214
Amortization of prior service cost, Net-of-Tax Amount	207	55	621	165	676	221
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,329	3,667	15,987	11,000	19,654	16,297
Prior service cost, Net-of-Tax Amount	0	0	0	0	(4,130)	0
Regulatory adjustment, Net-of-Tax Amount	(4,828)	(3,210)	(14,484)	(9,630)	82,137	(72,176)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	708	512	2,124	1,535	(9,324)	6,556
Amounts reclassified into net income	519	518	1,556	1,555	2,074	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	518	1,556	1,555	2,074	2,074
Translation adjustments, Net-of-Tax Amount	(510)	0	(1,573)	0	(2,232)	0
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(510)	0	(1,573)	0	(2,232)	0
Total other comprehensive income (loss), net of tax	$ 717	$ 1,030	$ 2,107	$ 3,090	$ (9,482)	$ 8,630