Equity, Other Comprehensive Income, and Accumulated Other Comprehensive Income - Rollforward of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Schedule Of Stockholders Equity [Line Items]
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans Before-Tax			$ (52,470)
Amortization of prior service costs, Defined Benefit Plans Before-Tax			266
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,544	6,398	25,634	19,198	32,033	23,091
Regulatory adjustment, Defined Benefit Plans Before-Tax	(7,583)	(5,847)	(22,751)	(17,544)	37,564	99,164
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax			3,149
Ending Balance AOCI September 30, 2013 Defined Benefit Plans Before-Tax	(49,321)		(49,321)		(49,321)
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans Tax (Expense) Benefit			19,939
Amortization of prior service costs, Defined Benefit Plans Tax (Expense) Benefit			(102)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit			(9,741)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,882	2,222	8,646	6,667	(14,274)	(37,682)
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	(399)	(292)	(1,197)	(876)	2,735	4,700
Ending Balance AOCI September 30, 2013 Defined Benefit Plans Tax (Expense) Benefit	18,742		18,742		18,742
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans After-Tax			(32,531)
Amortization of prior service costs, Defined Benefit Plans After-Tax			164
Amortization of net actuarial loss, Defined Benefit Plans After-Tax			15,893
Regulatory adjustment, AOCI	(4,701)	(3,625)	(14,105)	(10,877)	23,290	61,482
Net current period other comprehensive income (loss), Defined Benefit Plans After-Tax			1,952
Ending Balance AOCI September 30, 2013 Defined Benefit Plans After-Tax	(30,579)		(30,579)		(30,579)
Beginning Balance AOCI December 31, 2012, FSIRS - Before-Tax			(29,378)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	836	836	2,509	1,965	3,345	2,257
FSIRS other comprehensive income (loss), Before-Tax Amount	836	836	2,509	4,924	3,345	3,640
Ending Balance AOCI September 30, 2013, FSIRS - Before-Tax	(26,869)		(26,869)		(26,869)
Beginning Balance AOCI December 31, 2012, FSIRS - Tax (Expense) Benefit			11,164
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(318)	(318)	(954)	(747)	(1,271)	(858)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(318)	(318)	(954)	(1,872)	(1,271)	(1,384)
Ending Balance AOCI September 30, 2013, FSIRS - Tax (Expense) Benefit	10,210		10,210		10,210
Beginning Balance AOCI December 31, 2012, FSIRS - After-Tax			(18,214)
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	518	518	1,555	1,218	2,074	1,399
Net forward-starting interest rate swaps	518	518	1,555	3,052	2,074	2,256
Ending Balance AOCI September 30, 2013, FSIRS - After-Tax	(16,659)		(16,659)		(16,659)
Beginning Balance AOCI December 31, 2012, AOCI			(50,745)
Amounts reclassified into net income	518	518	1,555	1,218	2,074	1,399
Amortization of prior service costs, AOCI			164
Amortization of net actuarial loss, AOCI			15,893
Regulatory adjustment, Net-of-Tax Amount	(4,701)	(3,625)	(14,105)	(10,877)	23,290	61,482
Net current period other comprehensive income (loss), AOCI			3,507
Ending Balance AOCI September 30, 2013, AOCI	(47,238)		(47,238)		(47,238)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Schedule Of Stockholders Equity [Line Items]
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax
Other comprehensive income before reclassifications, FSIRS - Before-Tax
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax			2,509
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit			(954)
Other comprehensive income before reclassifications, FSIRS - After-Tax
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax			1,555
Other comprehensive income before reclassifications, AOCI
Amounts reclassified into net income			$ 1,555