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, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount			$ (173,646)	$ 100,345
Amortization of prior service cost, Before-Tax Amount	333	89	599	355
Amortization of net actuarial (gain)/loss, Before-Tax Amount	8,596	5,914	26,338	31,546
Prior service cost, Before-Tax Amount			(6,661)
Regulatory adjustment, Before-Tax Amount	(7,787)	(5,178)	137,699	(121,224)
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	1,142	825	(15,671)	11,022
Amounts reclassified into net income, Before-Tax Amount	836	836	3,345	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	3,345	3,345
Translation adjustments, Before-Tax Amount	(1,272)		(1,931)
Foreign currency other comprehensive income (loss), Before-Tax Amount	(1,272)		(1,931)
Total other comprehensive income (loss), Before-Tax Amount	706	1,661	(14,257)	14,367
Net actuarial gain/(loss), Tax (Expense) or Benefit			65,985	(38,131)
Amortization of prior service cost, Tax (Expense) or Benefit	(127)	(34)	(228)	(135)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(3,266)	(2,247)	(10,008)	(11,987)
Prior service cost, Tax (Expense) or Benefit			2,531
Regulatory adjustment, Tax (Expense) or Benefit	2,959	1,968	(52,326)	46,065
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(434)	(313)	5,954	(4,188)
Amounts reclassified into net income, Tax (Expense) or Benefit	(317)	(318)	(1,271)	(1,271)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(317)	(318)	(1,271)	(1,271)
Translation adjustments, Tax (Expense) or Benefit	0	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	(751)	(631)	4,683	(5,459)
Net actuarial gain/(loss), Net-of-Tax Amount			(107,661)	62,214
Amortization of prior service cost, Net-of-Tax Amount	206	55	371	220
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	5,330	3,667	16,330	19,559
Prior service cost, Net-of-Tax Amount			(4,130)
Regulatory adjustment, Net-of-Tax Amount	(4,828)	(3,210)	85,373	(75,159)
Pension plans other comprehensive income (loss), Net-of-Tax Amount	708	512	(9,717)	6,834
Amounts reclassified into net income	519	518	2,074	2,074
FSIRS other comprehensive income (loss), Net-of-Tax Amount	519	518	2,074	2,074
Translation adjustments, Net-of-Tax Amount	(1,272)		(1,931)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	(1,272)		(1,931)
Total other comprehensive income (loss), net of tax	$ (45)	$ 1,030	$ (9,574)	$ 8,908