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		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 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			177
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	8,545	6,400	17,090	12,800	29,887	20,585
Regulatory adjustment, Defined Benefit Plans Before-Tax	(7,584)	(5,848)	(15,168)	(11,697)	39,300	101,419
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax			2,099
Ending Balance AOCI June 30, 2013 Defined Benefit Plans Before-Tax	(50,371)		(50,371)		(50,371)
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			(68)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(3,247)	(2,432)	(6,494)	(4,864)	(11,357)	(7,822)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,882	2,222	5,764	4,445	(14,934)	(38,539)
Net current period other comprehensive income (loss), Defined Benefit Plans Tax (Expense) Benefit	(399)	(292)	(798)	(584)	2,842	4,795
Ending Balance AOCI June 30, 2013 Defined Benefit Plans Tax (Expense) Benefit	19,141		19,141		19,141
Beginning Balance AOCI December 31, 2012, Defined Benefit Plans After-Tax			(32,531)
Amortization of prior service costs, Defined Benefit Plans After-Tax			109
Amortization of net actuarial loss, Defined Benefit Plans After-Tax	5,298	3,968	10,596	7,936	18,530	12,763
Regulatory adjustment, AOCI	(4,702)	(3,626)	(9,404)	(7,252)	24,366	62,880
Net current period other comprehensive income (loss), Defined Benefit Plans After-Tax			1,301
Ending Balance AOCI June 30, 2013 Defined Benefit Plans After-Tax	(31,230)		(31,230)		(31,230)
Beginning Balance AOCI December 31, 2012, FSIRS - Before-Tax			(29,378)
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	837	837	1,673	1,129	3,345	1,713
FSIRS other comprehensive income (loss), Before-Tax Amount	837	837	1,673	4,088	3,345	(10,141)
Ending Balance AOCI June 30, 2013, FSIRS - Before-Tax	(27,705)		(27,705)		(27,705)
Beginning Balance AOCI December 31, 2012, FSIRS - Tax (Expense) Benefit			11,164
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(318)	(318)	(636)	(429)	(1,271)	(651)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(318)	(318)	(636)	(1,554)	(1,271)	3,853
Ending Balance AOCI June 30, 2013, FSIRS - Tax (Expense) Benefit	10,528		10,528		10,528
Beginning Balance AOCI December 31, 2012, FSIRS - After-Tax			(18,214)
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	519	519	1,037	700	2,074	1,062
Net forward-starting interest rate swaps	519	519	1,037	2,534	2,074	(6,288)
Ending Balance AOCI June 30, 2013, FSIRS - After-Tax	(17,177)		(17,177)		(17,177)
Beginning Balance AOCI December 31, 2012, AOCI			(50,745)
Amounts reclassified into net income	519	519	1,037	700	2,074	1,062
Amortization of prior service costs, AOCI			109
Amortization of net actuarial loss, AOCI			10,596
Regulatory adjustment, Net-of-Tax Amount	(4,702)	(3,626)	(9,404)	(7,252)	24,366	62,880
Net current period other comprehensive income (loss), AOCI			2,338
Ending Balance AOCI June 30, 2013, AOCI	(48,407)		(48,407)		(48,407)
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			1,673
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit			(636)
Other comprehensive income before reclassifications, FSIRS - After-Tax
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax			1,037
Other comprehensive income before reclassifications, AOCI
Amounts reclassified into net income			$ 1,037