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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount					$ (173,646)	$ 100,345
Amortization of prior service cost, Before-Tax Amount	$ 334	$ 88	$ 667	$ 177	845	355
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,595	5,914	17,191	11,828	29,019	28,915
Prior service cost, Before-Tax Amount					(6,661)	0
Regulatory adjustment, Before-Tax Amount	(7,787)	(5,177)	(15,574)	(10,355)	135,089	(118,817)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	2,284	1,650	(15,354)	10,798
Amounts reclassified into net income, Before-Tax Amount	836	837	1,672	1,673	3,344	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	837	1,672	1,673	3,344	3,345
Translation adjustments, Before-Tax Amount	209	0	(1,063)	0	(1,722)	0
Foreign currency other comprehensive income (loss), Before-Tax Amount	209	0	(1,063)	0	(1,722)	0
Total other comprehensive income (loss), Before-Tax Amount	2,187	1,662	2,893	3,323	(13,732)	14,143
Net actuarial gain/(loss), Tax (Expense) or Benefit					65,985	(38,131)
Amortization of prior service cost, Tax (Expense) or Benefit	(126)	(33)	(253)	(67)	(321)	(134)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(3,267)	(2,248)	(6,533)	(4,495)	(11,027)	(10,988)
Prior service cost, Tax (Expense) or Benefit					2,531	0
Regulatory adjustment, Tax (Expense) or Benefit	2,959	1,967	5,918	3,935	(51,334)	45,150
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(314)	(868)	(627)	5,834	(4,103)
Amounts reclassified into net income, Tax (Expense) or Benefit	(318)	(318)	(635)	(636)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(318)	(318)	(635)	(636)	(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	(752)	(632)	(1,503)	(1,263)	4,563	(5,374)
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	208	55	414	110	524	221
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,328	3,666	10,658	7,333	17,992	17,927
Prior service cost, Net-of-Tax Amount	0	0	0	0	(4,130)	0
Regulatory adjustment, Net-of-Tax Amount	(4,828)	(3,210)	(9,656)	(6,420)	83,755	(73,667)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	708	511	1,416	1,023	(9,520)	6,695
Amounts reclassified into net income	518	519	1,037	1,037	2,073	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	518	519	1,037	1,037	2,073	2,074
Translation adjustments, Net-of-Tax Amount	209	0	(1,063)	0	(1,722)	0
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	209	0	(1,063)	0	(1,722)	0
Total other comprehensive income (loss), net of tax	$ 1,435	$ 1,030	$ 1,390	$ 2,060	$ (9,169)	$ 8,769