Condensed Statement of Cash Flow (Parent Company) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statement of Cash Flows [Abstract]
Net loss	$ (36,740)	$ (1,279)	$ (21,513)
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Income (loss) from investment in the Partnership	3,250	0	0
Deferred tax provision (benefit)	(8,384)	(4,540)	(7,538)
Share-based Compensation	9,483	7,556	9,569
Changes in assets and liabilities:
Accounts receivable, accrued revenue and other	(39,120)	44,121	4,665
Accounts payable, accrued gas purchases and other accrued liabilities	19,744	(37,800)	18,996
Net cash provided by operating activities	100,456	141,293	84,790
Cash flows from investing activities:
Investment in limited liability company	(52,250)	(35,000)	0
Net cash provided by (used in) investing activities	(490,283)	(132,094)	14,638
Cash flows from financing activities:
Proceeds from exercise of common stock options	436	591	890
Conversion of restricted stock, net of shares withheld for taxes	(794)	(1,068)	(705)
Common dividends paid	(22,925)	(17,872)	(3,368)
Net cash provided by (used in) financing activities continuing operations	362,460	(1,636)	(87,351)
Net increase (decrease) in cash and cash equivalents	(27,367)	7,563	12,077
Cash and cash equivalents, beginning of period	30,343	22,780	10,703
Cash and cash equivalents, end of period	2,976	30,343	22,780
Parent Company [Member]
Statement of Cash Flows [Abstract]
Net loss	(12,481)	(6,007)	(11,651)
Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]
Income (loss) from investment in the Partnership	(16,080)	(7,192)	(16,042)
Deferred tax provision (benefit)	(7,367)	(3,896)	(7,142)
Share-based Compensation	277	249	292
Changes in assets and liabilities:
Accounts receivable, accrued revenue and other	27	57	6
Accounts payable, accrued gas purchases and other accrued liabilities	79	238	71
Net cash provided by operating activities	(3,439)	(2,277)	(2,395)
Cash flows from investing activities:
Investment in limited liability company	3,460	163	2,807
Distributions from the Partnership	27,270	22,497	4,435
Net cash provided by (used in) investing activities	23,810	22,334	1,628
Cash flows from financing activities:
Conversion of restricted stock, net of shares withheld for taxes	(794)	(1,068)	(705)
Common dividends paid	(22,925)	(17,872)	(3,368)
Net cash provided by (used in) financing activities continuing operations	(23,719)	(18,940)	(4,073)
Net increase (decrease) in cash and cash equivalents	(3,348)	1,117	(4,840)
Cash and cash equivalents, beginning of period	6,200	5,083	9,923
Cash and cash equivalents, end of period	$ 2,852	$ 6,200	$ 5,083