Financial Information by Reportable Segment (Detail) (USD $) In Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
OPERATING REVENUES:
Operating Revenues	$ 185,098		$ 168,468		$ 364,484		$ 329,756
INCOME BEFORE INCOME TAXES:
Income Before Income Taxes	66,749		57,399		133,510		111,103
NET INCOME:
Net income	42,996		36,301		84,998		70,505
TOTAL ASSETS
Assets	4,544,334				4,544,334				4,307,873
Regulated Operation
OPERATING REVENUES:
Operating Revenues	185,115		168,479		364,518		329,777
INCOME BEFORE INCOME TAXES:
Income Before Income Taxes	89,424		82,227		179,939		159,619
NET INCOME:
Net income	61,924	[1]	56,711	[1]	124,853	[1]	110,170	[1]
TOTAL ASSETS
Assets	4,399,933				4,399,933				4,180,485
Parent Company And Nonregulated Operating Subsidiaries
OPERATING REVENUES:
Operating Revenues	91		105		192		212
INCOME BEFORE INCOME TAXES:
Income Before Income Taxes	(22,675)		(24,828)		(46,429)		(48,516)
NET INCOME:
Net income	42,996		36,301		84,998		70,505
TOTAL ASSETS
Assets	2,854,545				2,854,545				2,762,210
Intercompany eliminations
OPERATING REVENUES:
Operating Revenues	(108)		(116)		(226)		(233)
NET INCOME:
Net income	(61,924)		(56,711)		(124,853)		(110,170)
TOTAL ASSETS
Assets	(2,724,768)				(2,724,768)				(2,622,944)
Reconciliations
TOTAL ASSETS
Assets	$ 14,624	[2]			$ 14,624	[2]			$ (11,878)	[2]

[1] Income tax provision and net income for our Regulated Operating Subsidiaries do not include any allocation of taxes for METC. METC is organized as a single-member limited liability company that is a disregarded entity for federal income tax purposes. METC is treated as a branch of MTH, which is taxed as a multiple-partner limited partnership for federal income tax purposes. Since METC and MTH, its immediate parent, file as a partnership for federal income tax purposes, they are exempt from federal income taxes. As a result, METC does not record a provision for federal income taxes in its statements of operations or record amounts for federal deferred income tax assets or liabilities on its statements of financial position. For FERC regulatory reporting, however, METC computes theoretical federal income taxes as well as the associated deferred income taxes and includes an annual allowance for income taxes in its net revenue requirement used to determine its rates.

[2] Reconciliation of total assets results primarily from differences in the netting of deferred tax assets and liabilities at our Regulated Operating Subsidiaries as compared to the classification in our condensed consolidated statements of financial position.