Segment Reporting (Summary of Financial Information Concerning Company's Reportable Segments) (Detail) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2018 USD ($)		Jun. 30, 2017 USD ($)		Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)
Segment Reporting Information [Line Items]
Revenue		$ 1,239,968		$ 1,175,569		$ 2,380,099	$ 2,266,835
Segment EBITDA		386,685	[1]	363,319	[1]	$ 735,706	682,154
Number of operating segments | segment						6
Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		1,417,946		1,342,475		$ 2,714,369	2,585,321
Segment EBITDA		386,425		374,481		741,789	708,937
Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(177,978)	[2]	(166,906)	[2]	(334,270)	(318,486)
Southern [Member]
Segment Reporting Information [Line Items]
Revenue		283,110		285,383		556,449	565,505
Segment EBITDA		68,787	[1]	67,168	[1]	137,694	136,108
Southern [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		321,051		323,108		630,007	640,447
Southern [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(37,941)	[2]	(37,725)	[2]	(73,558)	(74,942)
Western [Member]
Segment Reporting Information [Line Items]
Revenue		263,699		257,492		508,862	493,082
Segment EBITDA		81,175	[1]	87,045	[1]	153,832	162,613
Western [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		295,730		288,953		570,850	552,954
Western [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(32,031)	[2]	(31,461)	[2]	(61,988)	(59,872)
Eastern [Member]
Segment Reporting Information [Line Items]
Revenue		270,943		243,067		512,822	472,036
Segment EBITDA		74,795	[1]	70,668	[1]	142,229	135,297
Eastern [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		324,888		289,082		612,727	559,756
Eastern [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(53,945)	[2]	(46,015)	[2]	(99,905)	(87,720)
Canada [Member]
Segment Reporting Information [Line Items]
Revenue		186,838		183,540		356,813	349,094
Segment EBITDA		67,305	[1]	67,792	[1]	126,571	125,914
Canada [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		211,787		209,874		403,475	397,829
Canada [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(24,949)	[2]	(26,334)	[2]	(46,662)	(48,735)
Central [Member]
Segment Reporting Information [Line Items]
Revenue		175,020	[3]	159,418	[3]	328,809	303,727
Segment EBITDA		63,132	[1],[3]	60,716	[1],[3]	121,553	113,368
Central [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		202,725	[3]	182,781	[3]	377,925	346,593
Central [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(27,705)	[2],[3]	(23,363)	[2],[3]	(49,116)	(42,866)
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Revenue		60,358		46,669		116,344	83,391
Segment EBITDA		31,231	[1]	21,092	[1]	59,910	35,637
Exploration and Production [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue		61,765		48,677		119,385	87,742
Exploration and Production [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		(1,407)	[2]	(2,008)	[2]	(3,041)	(4,351)
Corporate [Member]
Segment Reporting Information [Line Items]
Revenue	[3]
Segment EBITDA		$ 260	[1],[3]	(11,162)	[1],[3]	$ (6,083)	$ (26,783)
Corporate [Member] | Reportable Segments [Member]
Segment Reporting Information [Line Items]
Revenue	[3]
Corporate [Member] | Intercompany Revenue [Member]
Segment Reporting Information [Line Items]
Revenue	[2],[3]

[1] For those items included in the determination of segment EBITDA, the accounting policies of the segments are the same as those described in the Company's most recent Annual Report on Form 10-K.

[2] Intercompany revenues reflect each segment's total intercompany sales, including intercompany sales within a segment and between segments. Transactions within and between segments are generally made on a basis intended to reflect the market value of the service.

[3] Corporate functions include accounting, legal, tax, treasury, information technology, risk management, human resources, training and other administrative functions. Amounts reflected are net of allocations to the six operating segments.