Southwest Gas (SWX) 10-K 28 Feb 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Feb. 15, 2017	Jun. 30, 2016
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	SWX
Entity Registrant Name	SOUTHWEST GAS CORP
Entity Central Index Key	92,416
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		47,546,825
Entity Public Float			$ 3,736,883,501

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Utility plant:
Gas plant	$ 6,193,564,000	$ 5,854,917,000
Less: accumulated depreciation	(2,172,966,000)	(2,084,007,000)
Acquisition adjustments, net	196,000	370,000
Construction work in progress	111,177,000	119,805,000
Net utility plant (Note 2)	4,131,971,000	3,891,085,000
Other property and investments (Note 1)	342,343,000	313,531,000
Current assets:
Cash and cash equivalents	28,066,000	35,997,000
Accounts receivable, net of allowances (Note 3)	285,145,000	314,512,000
Accrued utility revenue	76,200,000	74,700,000
Income taxes receivable, net	4,455,000	34,175,000
Deferred purchased gas costs (Note 4)	2,608,000	3,591,000
Prepaids and other current assets (Notes 1, 4, and 13)	136,833,000	95,199,000
Total current assets	533,307,000	558,174,000
Noncurrent assets:
Goodwill (Note 1)	139,983,000	126,145,000
Deferred income taxes (Note 12)	1,288,000	428,000
Deferred charges and other assets (Notes 2, 4, and 13)	432,234,000	469,322,000
Total noncurrent assets	573,505,000	595,895,000
Total assets	5,581,126,000	5,358,685,000
Capitalization:
Common stock, $1 par (authorized - 60,000,000 shares; issued and outstanding - 47,482,068 and 47,377,575 shares) (Note 11)	49,112,000	49,007,000
Additional paid-in capital	903,123,000	896,448,000
Accumulated other comprehensive income (loss), net (Note 5)	(48,008,000)	(50,268,000)
Retained earnings	759,263,000	699,221,000
Total Southwest Gas Corporation equity	1,663,490,000	1,594,408,000
Noncontrolling interest	(2,217,000)	(2,083,000)
Total equity	1,661,273,000	1,592,325,000
Redeemable noncontrolling interest (Note 16)	22,590,000	16,108,000
Long-term debt, less current maturities (Note 7)	1,549,983,000	1,551,204,000
Total capitalization	3,233,846,000	3,159,637,000
Commitments and contingencies (Note 9)
Current liabilities:
Current maturities of long-term debt (Note 7)	50,101,000	19,475,000
Short-term debt (Note 8)	0	18,000,000
Accounts payable	184,669,000	164,857,000
Customer deposits	72,296,000	72,631,000
Income taxes payable, net	1,909,000	940,000
Accrued general taxes	42,921,000	47,337,000
Accrued interest	17,939,000	16,173,000
Deferred purchased gas costs (Note 4)	90,476,000	45,601,000
Other current liabilities (Notes 2, 4, and 13)	168,064,000	150,031,000
Total current liabilities	628,375,000	535,045,000
Deferred income taxes and other credits:
Deferred income taxes and investment tax credits, net (Note 12)	840,653,000	769,445,000
Accumulated removal costs (Note 4)	308,000,000	303,000,000
Other deferred credits and other long-term liabilities (Notes 2, 4, 10, and 13)	570,252,000	591,558,000
Total deferred income taxes and other credits	1,718,905,000	1,664,003,000
Total capitalization and liabilities	$ 5,581,126,000	$ 5,358,685,000

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Common stock, par	$ 1	$ 1
Common stock, authorized	60,000,000	60,000,000
Common stock, issued	47,482,068	47,377,575
Common stock, outstanding	47,482,068	47,377,575

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues:
Gas operating revenues	$ 1,321,412	$ 1,454,639	$ 1,382,087
Construction revenues	1,139,078	1,008,986	739,620
Total operating revenues	2,460,490	2,463,625	2,121,707
Operating expenses:
Net cost of gas sold	397,121	563,809	505,356
Operations and maintenance	401,724	393,199	383,732
Depreciation and amortization	289,132	270,111	253,027
Taxes other than income taxes	52,376	49,393	47,252
Construction expenses	1,024,423	898,781	647,857
Total operating expenses	2,164,776	2,175,293	1,837,224
Operating income	295,714	288,332	284,483
Other income and (expenses):
Net interest deductions (Notes 7 and 8)	(73,660)	(71,879)	(72,069)
Other income (deductions)	9,469	2,879	7,107
Total other income and (expenses)	(64,191)	(69,000)	(64,962)
Income before income taxes	231,523	219,332	219,521
Income tax expense (Note 12)	78,468	79,902	78,373
Net income	153,055	139,430	141,148
Net income (loss) attributable to noncontrolling interests	1,014	1,113	22
Net income attributable to Southwest Gas Corporation	$ 152,041	$ 138,317	$ 141,126
Basic earnings per share (Notes 1 and 15)	$ 3.20	$ 2.94	$ 3.04
Diluted earnings per share (Notes 1 and 15)	$ 3.18	$ 2.92	$ 3.01
Average number of common shares outstanding	47,469	46,992	46,494
Average shares outstanding (assuming dilution)	47,814	47,383	46,944

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net Income	$ 153,055	$ 139,430	$ 141,148
Defined benefit pension plans (Notes 5 and 10):
Net actuarial gain (loss)	(14,118)	(18,922)	(107,661)
Amortization of prior service cost	828	828	220
Amortization of net actuarial loss	16,781	21,316	14,667
Prior service cost	0	0	(4,130)
Regulatory adjustment	(3,462)	(3,500)	86,991
Net defined benefit pension plans	29	(278)	(9,913)
Forward-starting interest rate swaps:
Amounts reclassified into net income (Notes 5 and 13)	2,075	2,073	2,073
Net forward-starting interest rate swaps	2,075	2,073	2,073
Foreign currency translation adjustments	161	(1,954)	(659)
Total other comprehensive income (loss), net of tax	2,265	(159)	(8,499)
Comprehensive income	155,320	139,271	132,649
Comprehensive income (loss) attributable to noncontrolling interests	1,019	1,047	0
Comprehensive income attributable to Southwest Gas Corporation	$ 154,301	$ 138,224	$ 132,649

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CASH FLOW FROM OPERATING ACTIVITIES:
Net Income	$ 153,055	$ 139,430	$ 141,148
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	289,132	270,111	253,027
Deferred income taxes	68,732	48,785	64,309
Changes in current assets and liabilities:
Accounts receivable, net of allowances	30,096	(39,850)	(3,683)
Accrued utility revenue	(1,500)	(800)	(1,200)
Deferred purchased gas costs	45,858	129,566	(69,339)
Accounts payable	21,695	(3,491)	(41,499)
Accrued taxes	26,340	(8,405)	(13,573)
Other current assets and liabilities	(29,551)	18,300	23,379
Gains on sale	(7,148)	(3,102)	(6,171)
Changes in undistributed stock compensation	5,456	2,914	7,973
AFUDC	(2,289)	(3,008)	(1,995)
Changes in other assets and deferred charges	16,960	(14,166)	(21,732)
Changes in other liabilities and deferred credits	(18,447)	10,863	15,779
Net cash provided by operating activities	598,389	547,147	346,423
CASH FLOW FROM INVESTING ACTIVITIES:
Construction expenditures and property additions	(529,531)	(488,000)	(396,898)
Acquisition of businesses, net of cash acquired	(17,000)	(9,261)	(190,497)
Restricted cash	0	785	1,233
Changes in customer advances	7,900	18,300	20,363
Miscellaneous inflows	13,039	8,354	11,611
Miscellaneous outflows	0	0	(1,400)
Net cash used in investing activities	(525,592)	(469,822)	(555,588)
CASH FLOW FROM FINANCING ACTIVITIES:
Issuance of common stock, net	472	35,396	405
Dividends paid	(83,317)	(74,248)	(66,275)
Centuri distribution to redeemable noncontrolling interest	(439)	(99)	0
Issuance of long-term debt, net	423,946	135,816	269,228
Retirement of long-term debt	(255,273)	(187,973)	(139,155)
Change in credit facility and commercial paper	(145,000)	0	140,000
Change in short-term debt	(18,000)	13,000	5,000
Principal payments on capital lease obligations	(1,354)	(1,420)	(434)
Other	(1,569)	41	(1,257)
Net cash provided by (used in) financing activities	(80,534)	(79,487)	207,512
Effects of currency translation on cash and cash equivalents	(194)	(1,407)	142
Change in cash and cash equivalents	(7,931)	(3,569)	(1,511)
Cash and cash equivalents at beginning of period	35,997	39,566	41,077
Cash and cash equivalents at end of period	28,066	35,997	39,566
Supplemental information:
Interest paid, net of amounts capitalized	67,440	66,623	65,552
Income taxes paid (received)	$ (19,032)	$ 43,225	$ 24,247

Consolidated Statements of Equi

Consolidated Statements of Equity and Redeemable Noncontrolling Interest - USD ($) $ in Thousands	Total	Common Stock [Member]		Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Non-controlling Interest [Member]	Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Balance at Dec. 31, 2013	$ 1,412,395	$ 47,986		$ 840,521	$ (41,698)	$ 567,714	$ (2,128)
Balance, Shares at Dec. 31, 2013		46,356,000
Common stock issuances, Value	11,027	$ 167		10,860
Common stock issuances, Shares		167,000
Redeemable noncontrolling interest attributable to acquisition								$ 18,952
Net income (loss)	140,997					141,126	(129)
Net income (loss) attributable to temporary equity								151
Redemption value adjustments (Note 16)	(961)					(961)
Redemption value adjustments attributable to temporary equity								961
Foreign currency exchange translation adj.	(637)				(637)
Foreign currency exchange translation adj attributable to temporary equity								(22)
Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax (Notes 5 and 10)	(9,913)				(9,913)
Amounts reclassified into net income (Notes 5 and 13)	2,073				2,073
Dividends declared Common	(68,715)					(68,715)
Balance at Dec. 31, 2014	1,486,266	$ 48,153		851,381	(50,175)	639,164	(2,257)
Balance, Shares at Dec. 31, 2014		46,523,000
Balance, Attributable to temporary equity								20,042
Common stock issuances, Value	40,144	$ 854		39,290
Common stock issuances, Shares		854,000
Net income (loss)	138,491					138,317	174
Net income (loss) attributable to temporary equity								939
Redemption value adjustments (Note 16)	4,708			5,777		(1,069)
Redemption value adjustments attributable to temporary equity								(4,708)
Foreign currency exchange translation adj.	(1,888)				(1,888)
Foreign currency exchange translation adj attributable to temporary equity								(66)
Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax (Notes 5 and 10)	(278)				(278)
Amounts reclassified into net income (Notes 5 and 13)	2,073				2,073
Centuri distribution to redeemable noncontrolling interest								(99)
Dividends declared Common	(77,191)					(77,191)
Balance at Dec. 31, 2015	$ 1,592,325	$ 49,007		896,448	(50,268)	699,221	(2,083)
Balance, Shares at Dec. 31, 2015	47,377,575	47,377,000
Balance, Attributable to temporary equity	$ 16,108							16,108
Common stock issuances, Value	6,780	$ 105		6,675
Common stock issuances, Shares		105,000
Net income (loss)	151,907					152,041	(134)
Net income (loss) attributable to temporary equity								1,148
Redemption value adjustments (Note 16)	(5,768)			0		(5,768)
Redemption value adjustments attributable to temporary equity								5,768
Foreign currency exchange translation adj.	156				156
Foreign currency exchange translation adj attributable to temporary equity								5
Net actuarial gain (loss) arising during the period, less amortization of unamortized benefit plan cost, net of tax (Notes 5 and 10)	29				29
Amounts reclassified into net income (Notes 5 and 13)	2,075				2,075
Centuri distribution to redeemable noncontrolling interest								(439)
Dividends declared Common	(86,231)					(86,231)
Balance at Dec. 31, 2016	$ 1,661,273	$ 49,112		$ 903,123	$ (48,008)	$ 759,263	$ (2,217)
Balance, Shares at Dec. 31, 2016	47,482,068	47,482,000	[1]
Balance, Attributable to temporary equity	$ 22,590							$ 22,590
[1]	There are 3.8 million common shares registered and available for issuance under provisions of the various stock issuance plans.

Consolidated Statements of Equ8

Consolidated Statements of Equity and Redeemable Noncontrolling Interest (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Stockholders' Equity [Abstract]
Dividends declared Common, per share	$ 1.80	$ 1.62	$ 1.46
Shares registered and available for issuance	3,800,000

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Note 1 — Summary of Significant Accounting Policies Holding Company Reorganization. one-for-one Nature of Operations. Acquisition of Construction Services Businesses Basis of Presentation. Consolidation. Centuri, through its subsidiaries, holds a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning (“HVAC”) technology and products. Centuri consolidates the entity (IntelliChoice Energy, LLC). Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction LTD. (“Western”), a Canadian construction services company that is a variable interest entity. Centuri determined that it is not the primary beneficiary of the entity due to a shared-power structure; therefore, Centuri does not consolidate the entity and has recorded its investment, and results related thereto, using the equity method. The investment in Western totaled $10.8 million at December 31, 2015 and 2016. Both periods include the impacts of foreign currency exchange translation adjustments. Dividends of $500,000 were received from Western during 2016 with no impact on earnings. No dividends were received from Western in 2015. A management fee was paid by Western to its partners, including W.S. Nicholls, in accordance with underlying agreements. The equity method investment in Western is included in Other Property and Investments in the Consolidated Balance Sheets. Centuri’s maximum exposure to loss as a result of its involvement with Western is estimated at $35.8 million. The estimated maximum exposure to loss represents the maximum loss that would be absorbed by Centuri in the event that all of the assets of Western were deemed to be worthless. Centuri recorded earnings of $69,000 from this investment in 2016, which is included in Other Income (deductions) in the Consolidated Statements of Income. In addition, Centuri, through its subsidiaries, has a 25% interest in CCI-TBN Net Utility Plant. Other Property and Investments. 2016 2015 Centuri property, equipment, and intangibles $ 451,114 $ 423,369 Centuri accumulated provision for depreciation and amortization (228,374 ) (221,028 ) Net cash surrender value of COLI policies 106,744 99,276 Other property 12,859 11,914 Total $ 342,343 $ 313,531 Deferred Purchased Gas Costs. Prepaids and other current assets Income Taxes. Cash and Cash Equivalents. Significant non-cash non-cash non-cash non-cash Goodwill. th Natural Construction Consolidated (In thousands of dollars) December 31, 2015 $ 10,095 $ 116,050 $ 126,145 Additional goodwill from ETTI acquisition — 10,726 10,726 Foreign currency translation adjustment — 3,112 3,112 December 31, 2016 $ 10,095 $ 129,888 $ 139,983 Goodwill from the ETTI acquisition consists of the excess of purchase price over the fair value of the acquired net assets and represents the value of the assembled workforce and the estimated economic value attributable to future opportunities that will arise based on the strong financial performance of the combined entities. Acquisition of Construction Services Businesses. Assets acquired in the transaction were recorded at their acquisition date fair values. The final purchase accounting is complete. The final estimated fair values of assets acquired as of May 6, 2016, the acquisition date, are as follows (in millions of dollars): Acquisition Property, plant and equipment $ 4.3 Intangible assets 2.9 Goodwill 10.7 Total assets acquired $ 17.9 The purchase price consisted of $17 million in cash on the acquisition date with the remaining amount being deferred over four years. Intangible Assets December 31, 2016 Gross Carrying Accumulated Net Carrying Customer relationships $ 34,033 $ (3,906 ) $ 30,127 Trade names and trademarks 9,349 (2,565 ) 6,784 Customer contracts backlog 1,656 (1,656 ) — Noncompete agreement 1,029 (271 ) 758 Total $ 46,067 $ (8,398 ) $ 37,669 December 31, 2015 Customer relationships $ 31,226 $ (2,070 ) $ 29,156 Trade names and trademarks 8,621 (1,331 ) 7,290 Customer contracts backlog 1,606 (1,606 ) — Noncompete agreement 437 (110 ) 327 Total $ 41,890 $ (5,117 ) $ 36,773 The intangible assets (other than goodwill and software-related intangibles) are included in Other property and investments in the Consolidated Balance Sheets. The estimated future amortization of the intangible assets for the next five years is as follows (in thousands): 2017 $ 3,339 2018 3,126 2019 2,463 2020 2,395 2021 2,269 See Note 2 – Utility Plant and Leases for additional information regarding natural gas operations intangible assets. Accumulated Removal Costs. Gas Operating Revenues. The Company acts as an agent for state and local taxing authorities in the collection and remission of a variety of taxes, including sales and use taxes and surcharges. These taxes are not included in gas operating revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities. Construction Revenues. percentage-of-completion Construction Expenses. job-related Net Cost of Gas Sold. Operations and Maintenance Expense. Depreciation and Amortization. Accumulated Removal Costs Allowance for Funds Used During Construction (“AFUDC”). 2016 2015 2014 (In thousands) AFUDC: Debt portion $ 1,175 $ 1,666 $ 1,228 Equity portion 2,289 3,008 1,995 AFUDC capitalized as part of utility plant $ 3,464 $ 4,674 $ 3,223 AFUDC rate 7.35 % 7.32 % 7.73 % Other Income (Deductions). 2016 2015 2014 Change in COLI policies $ 7,400 $ (500 ) $ 5,300 Interest income 1,849 2,173 2,602 Equity AFUDC 2,289 3,008 1,995 Foreign currency transaction gain (loss) (22 ) (824 ) (178 ) Equity in earnings of unconsolidated investment - Western 69 310 107 Miscellaneous income and (expense) (2,116 ) (1,288 ) (2,719 ) Total other income (deductions) $ 9,469 $ 2,879 $ 7,107 Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). These life insurance policies on members of management and other key employees are used by the Company to indemnify itself against the loss of talent, expertise, and knowledge, as well as to provide indirect funding for certain nonqualified benefit plans. Current tax regulations provide for tax-free Foreign Currency Translation. Earnings Per Share. 2016 2015 2014 (In thousands) Average basic shares 47,469 46,992 46,494 Effect of dilutive securities: Stock options 1 8 17 Performance shares 124 171 215 Restricted stock units 220 212 218 Average diluted shares 47,814 47,383 46,944 Recently Issued Accounting Standards Updates. one-year Management has substantially completed the evaluation of the sources of revenue and are currently assessing the effect of the new guidance on the financial position, results of operations and cash flows. The assessment is contingent, in part, upon the completion of deliberations currently in progress by the utility industry, notably in connection with efforts to produce an accounting guide intended to be developed by the American Institute of Certified Public Accountants (“AICPA”). In association with this undertaking, the AICPA formed a number of industry task forces, including a Power & Utilities (“P&U”) Task Force, on which Company personnel actively participate via formal membership. Industry representatives and organizations, the largest auditing firms, the AICPA’s Revenue Recognition Working Group and its Financial Reporting Executive Committee have undertaken, and continue to undertake, consideration of several items relevant to the utility industry. Where applicable or necessary, the FASB’s Transition Resource Group (TRG) is also participating. Currently, the industry is working to address several items including the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers and the accounting for funds received from third parties to partially or fully reimburse the cost of construction of an asset. Currently, a timeline for the resolution of these deliberations has not been established. Southwest is actively working with its peers in the rate-regulated natural gas industry and with the public accounting profession to conclude on the accounting treatment for several other issues that are not expected to be addressed by the P&U Task Force. As of December 31, 2016, the construction services segment has substantially completed the evaluation of sources of revenue and is currently assessing the effect of the new guidance on financial position, results of operations and cash flows. The principals of the new revenue recognition guidance are very similar to existing guidance for construction contractors. Similar to the P&U Task Force noted above, the AICPA formed the Engineering and Construction Contractors Task Force to assist the construction industry with implementing the new guidance. The accounting guide the AICPA intends to release is expected to provide implementation guidance related to several issues including 1) combining contracts and separating performance obligations; 2) estimating change orders, incentives, penalties, liquidated damages and other variable consideration items and 3) acceptable measures of progress when recognizing revenue over time. Given the uncertainty with respect to the conclusions that might arise from the deliberations on issues associated with both the natural gas and construction services segments, the Company is currently unable to determine the effect the new guidance will have on its financial position, results of operations, cash flows, business processes, or the transition method it will utilize to adopt the new guidance. In January 2016, the FASB issued the update “Financial Instruments – Overall (Subtopic 825-10): In February 2016, the FASB issued the update “Leases (Topic 842)”. Under the update, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: • A lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and • A right-of-use Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Though companies have historically been required to make disclosures regarding leases and of contractual obligations, leases (with terms longer than a year) will no longer exist off-balance In March 2016, the FASB issued the update “Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting”. The amendments are intended to improve the accounting for employee share-based payments and affect all organizations that issue share-based payment awards to their employees. The update requires the recording of all of the tax effects related to share-based payments at settlement (or expiration) through the income statement. Currently, tax benefits in excess of compensation cost (“windfalls”) are recorded in equity, and tax deficiencies (“shortfalls”) are recorded in equity to the extent of previous windfalls, and then recorded in the income statement. While the simplification will reduce some of the administrative complexities by eliminating the need to track a “windfall pool,” it will increase the volatility of income tax expense. The update also allows entities to withhold shares for the employee tax burden up to the employees’ maximum individual tax rate in the relevant jurisdiction without resulting in a liability classification of the award (currently such withholding is limited to the employer’s minimum statutory withholding). The update clarifies that all cash payments made to taxing authorities on the employees’ behalf for withheld shares should be presented as financing activities on the statement of cash flows. Also, the update requires all tax-related In June 2016, the FASB issued the update “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments”. The update amends guidance on reporting credit losses for financial assets held at amortized cost basis and available for sale debt securities. For assets held at amortized cost basis, the update eliminates the “probable” threshold for initial recognition of credit losses in current U.S. GAAP and, instead, requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset to present the net amount expected to be collected. For available for sale debt securities, credit losses should be measured in a manner similar to current U.S. GAAP, however the update will require that credit losses be presented as an allowance rather than as a write-down. This update affects entities holding financial assets and net investment in leases that are not accounted for at fair value through net income. The update affects loans, debt securities, trade receivables, net investments in leases, off-balance In August 2016, the FASB issued the update “Classification of Certain Cash Receipts and Cash Payments”. This update addresses the following specific cash flow issues: debt prepayment or debt extinguishment costs; settlement of zero-coupon In October 2016, the FASB issued the update “Accounting for Income Taxes: Intra-Entity Asset Transfers of Assets Other than Inventory.” This update eliminates the current U.S. GAAP exception for all intra-entity sales of assets other than inventory. As a result, a reporting entity would recognize the tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs, even though the pre-tax In October 2016, the FASB issued the update “Consolidation (Topic 810): Interests Held through Related Parties That Are under Common Control.” The amendments affect reporting entities that are required to evaluate whether they should consolidate a variable interest entity in certain situations involving entities under common control. The update is effective for fiscal and interim periods beginning after December 15, 2016. Management has determined that this update is not impactful to its consolidated financial statements. In January 2017, the FASB issued the update “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” The update eliminates Step 2 from the goodwill impairment test. The annual, or interim, goodwill impairment test is performed by comparing the fair value of a reporting unit with its carrying amount. An impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. In addition, income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit should be considered when measuring the goodwill impairment loss, if applicable. The update also eliminates the requirements for any reporting unit with a zero or negative carrying amount to perform a qualitative assessment and, if it fails that qualitative test, to perform Step 2 of the goodwill impairment test. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. The amendments should be applied on a prospective basis. The update is effective for fiscal and interim periods beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. Management has determined that this update would have had no impact on the consolidated financial statements for the periods presented if it had been effective during those periods. Subsequent Events.

Utility Plant and Leases

Utility Plant and Leases	12 Months Ended
	Dec. 31, 2016
Leases [Abstract]
Utility Plant and Leases	Note 2 – Utility Plant and Leases Net utility plant as of December 31, 2016 and 2015 was as follows (thousands of dollars): December 31, 2016 2015 Gas plant: Storage $ 24,614 $ 22,944 Transmission 349,981 312,996 Distribution 5,198,531 4,935,730 General 382,084 365,865 Software and software-related intangibles 224,260 203,323 Other 14,094 14,059 6,193,564 5,854,917 Less: accumulated depreciation (2,172,966 ) (2,084,007 ) Acquisition adjustments, net 196 370 Construction work in progress 111,177 119,805 Net utility plant $ 4,131,971 $ 3,891,085 Utility plant depreciation is computed on the straight-line remaining life method at composite rates considered sufficient to amortize costs over estimated service lives, including components which compensate for removal costs (net of salvage value), and retirements, based on the processes of regulatory proceedings and related regulatory commission approvals and/or mandates. In 2016, annual depreciation and amortization expense averaged 3.6% of the original cost of depreciable and amortizable property. Average rates in 2015 and 2014 also approximated 3.6%. Transmission and Distribution plant (combined), associated with our core natural gas delivery infrastructure, constitute the majority of gas plant. Annual depreciation expense averaged approximately 3.3% of original cost of depreciable transmission and distribution plant during the period 2014 through 2016. Depreciation and amortization expense on gas plant, including intangibles, was as follows (thousands of dollars): 2016 2015 2014 Depreciation and amortization expense $ 214,037 $ 201,233 $ 194,360 Included in the figures above is amortization of intangibles of $14.8 million in 2016, $12.7 million in 2015, and $11.7 million in 2014. Operating Leases and Rentals. 2016 2015 2014 Southwest Gas $ 4,357 $ 4,186 $ 5,330 Centuri 53,956 45,849 30,012 Consolidated rental payments/lease expense $ 58,313 $ 50,035 $ 35,342 The following is a schedule of future minimum lease payments for significant non-cancelable Year Ending December 31, 2017 $ 6,929 2018 4,837 2019 3,449 2020 2,411 2021 1,098 Thereafter 2,730 Total minimum lease payments $ 21,454 Capital Leases. December 31, 2016 2015 Capital leased assets, gross $ 3,189 $ 4,584 Less: accumulated amortization (1,172 ) (1,043 ) Capital leased assets, net $ 2,017 $ 3,541 The following is a schedule of future minimum lease payments for non-cancelable Year Ending December 31, 2017 $ 931 2018 546 2019 84 2020 — 2021 — Thereafter — 1,561 Less: amount representing interest (101 ) Total minimum lease payments $ 1,460

Receivables and Related Allowan

Receivables and Related Allowances	12 Months Ended
	Dec. 31, 2016
Receivables [Abstract]
Receivables and Related Allowances	Note 3 – Receivables and Related Allowances Business activity with respect to gas utility operations is conducted with customers located within the three-state region of Arizona, Nevada, and California. The table below contains information about the gas utility customer accounts receivable balance (net of allowance) at December 31, 2016 and 2015, and the percentage of customers in each of the three states. December 31, December 31, Gas utility customer accounts receivable balance (in thousands) $ 111,320 $ 151,775 December 31, Percent of customers by state Arizona 53 % Nevada 37 % California 10 % Although Southwest seeks to minimize its credit risk related to utility operations by requiring security deposits from new customers, imposing late fees, and actively pursuing collection on overdue accounts, some accounts are ultimately not collected. Customer accounts are subject to collection procedures that vary by jurisdiction (late fee assessment, noticing requirements for disconnection of service, and procedures for actual disconnection and/or reestablishment of service). After disconnection of service, accounts are generally written off approximately one month after inactivation. Dependent upon the jurisdiction, reestablishment of service requires both payment of previously unpaid balances and additional deposit requirements. Provisions for uncollectible accounts are recorded monthly based on experience, customer and rate composition, and write-off Allowance for Balance, December 31, 2013 $ 1,725 Additions charged to expense 4,146 Accounts written off, less recoveries (3,616 ) Balance, December 31, 2014 2,255 Additions charged to expense 4,113 Accounts written off, less recoveries (4,098 ) Balance, December 31, 2015 2,270 Additions charged to expense 3,264 Accounts written off, less recoveries (3,010 ) Balance, December 31, 2016 $ 2,524 At December 31, 2016, the construction services segment (Centuri) had $173 million in customer accounts receivable. Both the allowance for uncollectibles and write-offs related to Centuri customers have been insignificant and are not reflected in the table above.

Regulatory Assets and Liabiliti

Regulatory Assets and Liabilities	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Regulatory Assets and Liabilities	Note 4 – Regulatory Assets and Liabilities Southwest is subject to the regulation of the Arizona Corporation Commission (“ACC”), the Public Utilities Commission of Nevada (“PUCN”), the California Public Utilities Commission (“CPUC”), and the Federal Energy Regulatory Commission (“FERC”). Accounting policies of Southwest conform to U.S. GAAP applicable to rate-regulated entities and reflect the effects of the ratemaking process. Accounting treatment for rate-regulated entities allows for deferral as regulatory assets, costs that otherwise would be expensed, if it is probable that future recovery from customers will occur. If rate recovery is no longer probable, due to competition or the actions of regulators, Southwest is required to write-off The following table represents existing regulatory assets and liabilities (thousands of dollars): December 31, 2016 2015 Regulatory assets: Accrued pension and other postretirement benefit costs (1) $ 379,063 $ 384,647 Unrealized net loss on non-trading — 5,486 Deferred purchased gas costs (3) 2,608 3,591 Accrued purchased gas costs (4) 37,100 — Unamortized premium on reacquired debt (5) 21,975 21,511 Accrued absence time (9) 13,440 13,240 Other (6) 23,557 59,782 477,743 488,257 Regulatory liabilities: Deferred purchased gas costs (3) (90,476 ) (45,601 ) Accumulated removal costs (308,000 ) (303,000 ) Accrued purchased gas costs (4) — (10,400 ) Unrealized net gain on non-trading (4,377 ) — Unamortized gain on reacquired debt (7) (9,789 ) (10,325 ) Other (8) (24,659 ) (36,631 ) Net regulatory assets $ 40,442 $ 82,300 (1) Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 10). (2) The following table details the regulatory assets/(liabilities) offsetting the derivatives (Swaps) at fair value in the Consolidated Balance Sheets (thousands of dollars). The actual amounts, when realized at settlement, become a component of purchased gas costs under Southwest’s purchased gas adjustment (“PGA”) mechanisms. (See Note 13). Instrument Balance Sheet Location 2016 2015 Swaps Deferred charges and other assets $ — $ 1,219 Swaps Prepaids and other current assets — 4,267 Swaps Other current liabilities (3,532 ) — Swaps Other deferred credits (845 ) — (3) Balance recovered or refunded on an ongoing basis with interest. (4) Asset included in Prepaids and other current assets and liability included in Other current liabilities on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis. (5) Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments. (6) The following table details the components of Other regulatory assets which are included in either Prepaids and other current assets or Deferred charges and other assets on the Consolidated Balance Sheets (as indicated). Recovery periods vary. Margin tracking/decoupling mechanisms are alternative revenue programs and revenue associated with under-collections (for the difference between authorized margin levels and amounts billed to customers through rates currently) are recognized as revenue so long as recovery is expected to take place within 24 months. Other Regulatory Assets 2016 2015 State mandated public purpose programs (including low income and conservation programs) (a) (f) $ 7,096 $ 18,101 Margin and interest-tracking accounts (b) (f) 3,517 30,339 Infrastructure replacement programs and similar (c) (f) 6,976 6,947 Environmental compliance programs (d) (f) 4,329 2,300 Other (e) 1,639 2,095 $ 23,557 $ 59,782 a) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets; 2015 included in Deferred charges and other assets on the Consolidated Balance Sheets. b) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets; 2015 included in Prepaids and other current assets on the Consolidated Balance Sheets ($11 million) and Deferred charges and other assets on the Consolidated Balance Sheets ($19.3 million). c) Included in Deferred charges and other assets on the Consolidated Balance Sheets with the exception of $6,000 in 2016 that is included in Prepaids and other current assets on the Consolidated Balance Sheets. d) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets ($3.8 million) and Deferred charges and other assets on the Consolidated Balance Sheets ($500,000); 2015 included in Prepaids and other current assets on the Consolidated Balance Sheets ($1.8 million) and Deferred charges and other assets on the Consolidated Balance Sheets ($484,000). e) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets ($622,000) and Deferred charges and other assets on the Consolidated Balance Sheets ($1 million); 2015 included in Deferred charges and other assets on the Consolidated Balance Sheets. f) Balance recovered or refunded on an ongoing basis, generally with interest. (7) Included in Other deferred credits on the Consolidated Balance Sheets. Amortized over life of debt instruments. (8) The following table details the components of Other regulatory liabilities which are included in either Other current liabilities or Deferred credits and other liabilities on the Consolidated Balance Sheets (as indicated). Other Regulatory Liabilities 2016 2015 State mandated public purpose programs (including low income and conservation programs) (a) (d) $ (7,101 ) $ (4,888 ) Margin and interest-tracking accounts (a) (d) (3,668 ) (20,191 ) Environmental compliance programs (b) (d) (4,469 ) (2,252 ) Regulatory offsets to deferred tax balances (c) (3,390 ) (4,866 ) Regulatory accounts for differences related to pension funding (c) (2,284 ) (1,363 ) Income tax and gross-up (c) (3,203 ) (3,067 ) Other (d) (e) (544 ) (4 ) $ (24,659 ) $ (36,631 ) a) 2016 included in Other current liabilities on the Consolidated Balance Sheets; 2015 included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. b) Included in Other current liabilities on the Consolidated Balance Sheets. c) Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. d) Balance recovered or refunded on an ongoing basis, generally with interest. e) 2016 included in Other current liabilities on the Consolidated Balance Sheets ($536,000) and in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets ($8,000); 2015 included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. (9) Regulatory recovery occurs on a one-year

Other Comprehensive Income and

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI")	Note 5 – Other Comprehensive Income and Accumulated Other Comprehensive Income (“AOCI”) The following information provides insight into amounts impacting Other Comprehensive Income (Loss), both before and after-tax, Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) 2016 2015 2014 Before- Tax Amount Tax (Expense) or Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Net actuarial gain/(loss) $ (22,770 ) $ 8,652 $ (14,118 ) $ (30,519 ) $ 11,597 $ (18,922 ) $ (173,646 ) $ 65,985 $ (107,661 ) Amortization of prior service cost 1,335 (507 ) 828 1,335 (507 ) 828 355 (135 ) 220 Amortization of net actuarial (gain)/loss 27,066 (10,285 ) 16,781 34,381 (13,065 ) 21,316 23,656 (8,989 ) 14,667 Prior service cost — — — — — — (6,661 ) 2,531 (4,130 ) Regulatory adjustment (5,584 ) 2,122 (3,462 ) (5,646 ) 2,146 (3,500 ) 140,308 (53,317 ) 86,991 Pension plans other comprehensive income (loss) 47 (18 ) 29 (449 ) 171 (278 ) (15,988 ) 6,075 (9,913 ) Forward-starting interest rate swaps (“FSIRS”) (designated hedging activities): Amounts reclassified into net income 3,345 (1,270 ) 2,075 3,344 (1,271 ) 2,073 3,345 (1,272 ) 2,073 FSIRS other comprehensive income (loss) 3,345 (1,270 ) 2,075 3,344 (1,271 ) 2,073 3,345 (1,272 ) 2,073 Foreign currency translation adjustments: Translation adjustments 161 — 161 (1,954 ) — (1,954 ) (659 ) — (659 ) Foreign currency other comprehensive income (loss) 161 — 161 (1,954 ) — (1,954 ) (659 ) — (659 ) Total other comprehensive income (loss) $ 3,553 $ (1,288 ) $ 2,265 $ 941 $ (1,100 ) $ (159 ) $ (13,302 ) $ 4,803 $ (8,499 ) (1) Tax amounts are calculated using a 38% rate. Management has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, management is not recognizing any tax effect or presenting a tax expense or benefit for the currency translation adjustment amount reported in Other Comprehensive Income, as repatriation of earnings is not anticipated. The estimated amounts that will be amortized from accumulated other comprehensive income or regulatory assets into net periodic benefit cost over the next year are summarized below (in thousands): Retirement plan net actuarial loss $ 24,000 SERP net actuarial loss 1,500 PBOP prior service cost 1,300 Approximately $2.1 million of realized losses (net of tax) related to the FSIRS, included in AOCI at December 31, 2016, will be reclassified into interest expense within the next twelve months as the related interest payments on long-term debt occur. The following table represents a rollforward of AOCI, presented on the Company’s Consolidated Balance Sheets and its Consolidated Statements of Equity: AOCI—Rollforward (Thousands of dollars) Defined Benefit Plans (Note 10) FSIRS (Note 13) Foreign Currency Items Before- Tax Tax After- Tax Before- Tax Tax After- Tax Before- Tax Tax After- Tax AOCI Beginning Balance AOCI December 31, 2015 $ (57,660 ) $ 21,911 $ (35,749 ) $ (19,344 ) $ 7,350 $ (11,994 ) $ (2,525 ) $ — $ (2,525 ) $ (50,268 ) Net actuarial gain/(loss) (22,770 ) 8,652 (14,118 ) — — — — — — (14,118 ) Translation adjustments — — — — — — 161 — 161 161 Other comprehensive income before reclassifications (22,770 ) 8,652 (14,118 ) — — — 161 — 161 (13,957 ) FSIRS amounts reclassified from AOCI (1) — — — 3,345 (1,270 ) 2,075 — — — 2,075 Amortization of prior service cost (2) 1,335 (507 ) 828 — — — — — — 828 Amortization of net actuarial loss (2) 27,066 (10,285 ) 16,781 — — — — — — 16,781 Regulatory adjustment (3) (5,584 ) 2,122 (3,462 ) — — — — — — (3,462 ) Net current period other comprehensive income (loss) 47 (18 ) 29 3,345 (1,270 ) 2,075 161 — 161 2,265 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — 5 — 5 5 Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 47 (18 ) 29 3,345 (1,270 ) 2,075 156 — 156 2,260 Ending Balance AOCI December 31, 2016 $ (57,613 ) $ 21,893 $ (35,720 ) $ (15,999 ) $ 6,080 $ (9,919 ) $ (2,369 ) $ — $ (2,369 ) $ (48,008 ) (1) The FSIRS reclassification amounts are included in the Net interest deductions line item on the Consolidated Statements of Income. (2) These AOCI components are included in the computation of net periodic benefit cost (see Note 10 – Pension and Other Postretirement Benefits (3) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Consolidated Balance Sheets). (4) Tax amounts are calculated using a 38% rate. The following table represents amounts (before income tax impacts) included in Accumulated other comprehensive income (in the table above), that have not yet been recognized in net periodic benefit cost as of December 31, 2016 and 2015: Amounts Recognized in AOCI (Before Tax) (Thousands of dollars) 2016 2015 Net actuarial (loss) gain $ (430,973 ) $ (435,269 ) Prior service cost (5,703 ) (7,038 ) Less: amount recognized in regulatory assets 379,063 384,647 Recognized in AOCI $ (57,613 ) $ (57,660 ) See Note 10 – Pension and Other Postretirement Benefits for more information on the defined benefit pension plans and Note 13 – Derivatives and Fair Value Measurements for more information on the FSIRS.

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2016
Text Block [Abstract]
Common Stock	Note 6 – Common Stock On March 10, 2015, the Company filed with the Securities Exchange Commission (“SEC”) an automatic shelf registration statement on Form S-3 No. 333-202633), at-the-market During 2016, the Company issued approximately 105,000 shares of common stock through the Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan.

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Long-Term Debt	Note 7 – Long-Term Debt Carrying amounts of the Company’s long-term debt and their related estimated fair values as of December 31, 2016 and December 31, 2015 are disclosed in the following table. The fair values of the revolving credit facility (including commercial paper) and the variable-rate Industrial Development Revenue Bonds (“IDRBs”) approximate their carrying values, as they are repaid quickly (in the case of credit facility borrowings) and have interest rates that reset frequently. They are categorized as Level 1 (quoted prices for identical financial instruments) within the three-level December 31, 2016 2015 Carrying Amount Market Value Carrying Amount Market Value (Thousands of dollars) Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 129,703 $ 125,000 $ 130,273 Notes, 6.1%, due 2041 125,000 149,734 125,000 141,581 Notes, 3.875%, due 2022 250,000 254,900 250,000 253,600 Notes, 4.875%, due 2043 250,000 266,793 250,000 251,483 Notes, 3.8%, due 2046 300,000 283,029 — — 8% Series, due 2026 75,000 94,691 75,000 97,035 Medium-term notes, 7.59% series, due 2017 25,000 25,040 25,000 26,253 Medium-term notes, 7.78% series, due 2022 25,000 29,290 25,000 29,855 Medium-term notes, 7.92% series, due 2027 25,000 31,905 25,000 31,890 Medium-term notes, 6.76% series, due 2027 7,500 8,769 7,500 8,684 Unamortized discount and debt issuance costs (9,931 ) (6,137 ) 1,197,569 901,363 Revolving credit facility and commercial paper 5,000 5,000 150,000 150,000 Industrial development revenue bonds: Variable-rate bonds: Tax-exempt 50,000 50,000 50,000 50,000 2003 Series A, due 2038 50,000 50,000 50,000 50,000 2008 Series A, due 2038 50,000 50,000 50,000 50,000 2009 Series A, due 2039 50,000 50,000 50,000 50,000 Fixed-rate bonds: 4.85% 2005 Series A, due 2035 — — 100,000 100,452 4.75% 2006 Series A, due 2036 — — 24,855 25,130 Unamortized discount and debt issuance costs (2,489 ) (3,946 ) 197,511 320,909 Centuri term loan facility 106,700 106,819 112,571 112,665 Unamortized debt issuance costs (516 ) (692 ) 106,184 111,879 Centuri secured revolving credit facility 41,185 41,292 60,627 60,724 Centuri other debt obligations 52,635 52,840 25,901 26,059 1,600,084 1,570,679 Less: current maturities (50,101 ) (19,475 ) Long-term debt, less current maturities $ 1,549,983 $ 1,551,204 In March 2016, the Company amended its $300 million credit and commercial paper facility. The facility was previously scheduled to expire in March 2020, but was extended to March 2021. The Company will continue to use $150 million of the facility as long-term debt and the remaining $150 million for working capital purposes. Interest rates for the credit facility are calculated at either the London Interbank Offered Rate (“LIBOR”) or an “alternate base rate,” plus in each case an applicable margin that is determined based on the Company’s senior unsecured debt rating. At December 31, 2016, the applicable margin is 1% for loans bearing interest with reference to LIBOR and 0% for loans bearing interest with reference to the alternative base rate. At December 31, 2016, $5 million was outstanding on the long-term portion of the credit facility, none of which was in commercial paper (see commercial paper program discussion below). The effective interest rate on the long-term portion of the credit facility was 5.21% at December 31, 2016. Borrowings under the credit facility ranged from none at various times throughout 2016 to a high of $230 million during the third quarter of 2016. With regard to the short-term portion of the credit facility, there were no borrowings outstanding at December 31, 2016 and $18 million outstanding at December 31, 2015. (See Note 8 – Short-Term Debt ). The Company has a $50 million commercial paper program. Any issuance under the commercial paper program is supported by the Company’s current revolving credit facility and, therefore, does not represent additional borrowing capacity. Any borrowing under the commercial paper program will be designated as long-term debt. Interest rates for the program are calculated at the then current commercial paper rate. At December 31, 2016, and as noted above, no borrowings were outstanding under the commercial paper program. Southwest redeemed its $100 million 2005 4.85% Series A fixed-rate IDRBs (originally due in 2035) at par with accrued interest in July 2016. In September 2016, Southwest redeemed its $24.9 million 2006 Series A 4.75% fixed-rate IDRBs (originally due in 2036) at par with accrued interest. In January 2017, subsequent to the most recent balance sheet date, the $25 million 7.59% medium-term notes were repaid at maturity, using available cash on hand. In September 2016, Southwest issued $300 million in 3.8% Senior Notes at a discount of 0.302%. The notes will mature in September 2046. A portion of the net proceeds were used to temporarily pay down amounts then outstanding under the credit facility. The remaining net proceeds were used for general corporate purposes. Centuri has a $300 million secured revolving credit and term loan facility that is scheduled to expire in October 2019. This facility includes a revolving credit facility and a term loan facility. The term loan facility had an initial limit of approximately $150 million, which was reached in 2014 and is in the process of being repaid. No further borrowing is permitted under the term loan facility. The revolving credit facility has a limit of $150 million; amounts borrowed and repaid under the revolving credit facility are available to be re-borrowed. Interest rates for Centuri’s $300 million secured revolving credit and term loan facility are calculated at the LIBOR, the Canadian Dealer Offered Rate (“CDOR”), or an alternate base rate or Canadian base rate, plus in each case an applicable margin that is determined based on Centuri’s consolidated leverage ratio. The applicable margin ranges from 1.00% to 2.25% for loans bearing interest with reference to LIBOR or CDOR and from 0.00% to 1.25% for loans bearing interest with reference to the alternate base rate or Canadian base rate. Centuri is also required to pay a commitment fee on the unfunded portion of the commitments based on their consolidated leverage ratio. The commitment fee ranges from 0.15% to 0.40% per annum. Borrowings under the revolving credit facility ranged from a low of $36.2 million during February 2016 to a high of $83.2 million during July 2016. All amounts outstanding are considered long-term borrowings. The effective interest rate on the secured revolving credit and term loan facility was 2.63% at December 31, 2016. The effective interest rates on Southwest’s variable-rate IDRBs are included in the table below: December 31, December 31, 2003 Series A 1.47 % 0.87 % 2008 Series A 1.53 % 0.87 % 2009 Series A 1.43 % 0.75 % Tax-exempt 1.51 % 0.81 % In Nevada, interest fluctuations due to changing interest rates on Southwest’s 2003 Series A, 2008 Series A, and 2009 Series A variable-rate IDRBs are tracked and recovered from ratepayers through an interest balancing account. None of Southwest’s debt instruments have credit triggers or other clauses that result in default if bond ratings are lowered by rating agencies. Certain debt instruments contain securities ratings covenants that, if set in motion, would increase financing costs. Certain debt instruments also have leverage ratio caps and minimum net worth requirements. At December 31, 2016, the Company is in compliance with all of its covenants. Under the most restrictive of the covenants, at December 31, 2016, approximately $2.3 billion in additional debt could be issued while still meeting the leverage ratio requirement. Relating to the minimum net worth requirement, as of December 31, 2016, there is at least $1.1 billion of cushion in equity. Certain Centuri debt instruments also have leverage ratio caps and fixed charge ratio coverage requirements. At December 31, 2016, Centuri is in compliance with all of its covenants. Under the most restrictive of the covenants, Centuri could issue approximately $145 million in additional debt and meet the leverage ratio requirement. Centuri has at least $21 million of cushion relating to the minimum fixed charge ratio coverage requirement. Estimated maturities of long-term debt for the next five years are (in thousands): 2017 $ 50,101 2018 24,082 2019 134,534 2020 134,452 2021 7,815

Short-Term Debt

Short-Term Debt	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Short-Term Debt	Note 8 – Short-Term Debt As discussed in Note 7, Southwest has a $300 million credit facility that is scheduled to expire in March 2021, of which $150 million has been designated by management for working capital purposes. Southwest had no short-term borrowings outstanding at December 31, 2016 and $18 million in short-term borrowings outstanding at December 31, 2015.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Note 9 – Commitments and Contingencies The Company is a defendant in miscellaneous legal proceedings. The Company is also a party to various regulatory proceedings. The ultimate dispositions of these proceedings are not presently determinable; however, it is the opinion of management that no litigation or regulatory proceeding to which the Company is currently subject will have a material adverse impact on its financial position, results of operations, or cash flows. Southwest maintains liability insurance for various risks associated with the operation of its natural gas pipelines and facilities. In connection with these liability insurance policies, Southwest is responsible for an initial deductible or self-insured retention amount per incident, after which the insurance carriers would be responsible for amounts up to the policy limits. For the policy year August 2016 to July 2017, these liability insurance policies require Southwest to be responsible for the first $1 million (self-insured retention) of each incident plus the first $4 million in aggregate claims above its self-insured retention in the policy year. Through an assessment process, Southwest may determine that certain costs are likely to be incurred in the future related to specific legal matters. In these circumstances and in accordance with accounting policies, Southwest will make an accrual, as necessary.

Pension and Other Postretiremen

Pension and Other Postretirement Benefits	12 Months Ended
	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
Pension and Other Postretirement Benefits	Note 10 – Pension and Other Postretirement Benefits An Employees’ Investment Plan is offered to eligible employees of Southwest through deduction of a percentage of base compensation, subject to IRS limitations. The Employees’ Investment Plan provides for purchases of various mutual fund investments and Company common stock. One-half 2016 2015 2014 Employee Investment Plan cost $ 4,976 $ 5,072 $ 4,816 Centuri has a separate plan, the cost and liability of which are not significant. A deferred compensation plan is offered to all officers and a separate deferred compensation plan for members of the Board of Directors. The plans provide the opportunity to defer up to 100% of annual cash compensation. One-half A noncontributory qualified retirement plan with defined benefits covering substantially all Southwest employees is available in addition to a separate unfunded supplemental executive retirement plan (“SERP”) which is limited to officers. Postretirement benefits other than pensions (“PBOP”) are provided to qualified retirees for health care, dental, and life insurance benefits. The overfunded or underfunded positions of defined benefit postretirement plans, including pension plans, are recognized in the Consolidated Balance Sheets. Any actuarial gains and losses, prior service costs and transition assets or obligations are recognized in Accumulated other comprehensive income under Stockholders’ equity, net of tax, until they are amortized as a component of net periodic benefit cost. A regulatory asset has been established for the portion of the total amounts otherwise chargeable to accumulated other comprehensive income that are expected to be recovered through rates in future periods. Changes in actuarial gains and losses and prior service costs pertaining to the regulatory asset will be recognized as an adjustment to the regulatory asset account as these amounts are amortized and recognized as components of net periodic pension costs each year. Investment objectives and strategies for the qualified retirement plan are developed and approved by the Pension Plan Investment Committee of the Board of Directors of the Company. They are designed to enhance capital, maintain minimum liquidity required for retirement plan operations, manage funded status risk and effectively manage pension assets. A target portfolio of investments in the qualified retirement plan is developed by the Pension Plan Investment Committee and is reevaluated periodically. Asset return assumptions are determined by evaluating performance expectations of the target portfolio. Projected benefit obligations are estimated using actuarial assumptions and Company benefit policy. A target mix of assets is then determined based on acceptable risk versus estimated returns in order to fund the benefit obligation. At December 31, 2016, the percentage ranges of the target portfolio are: Type of Investment Percentage Range Equity securities 63 to 67 Debt securities 33 to 37 Other up to 1 The qualified retirement plan invests the majority of its plan assets in common collective trusts which includes a well-diversified portfolio of domestic and international equity securities and fixed income securities, which are managed by a professional investment manager appointed by the Company. The investment manager has full discretionary authority to direct the investment of plan assets held in trust within the specific guidelines prescribed by the Company through the plan’s investment policy statement. In 2016, the Company adopted a liability driven investment (“LDI”) strategy for part of the portfolio, a form of investing designed to better match the movement in pension plan assets with the impact of interest rate changes and inflation assumption changes on the pension plan liability. The implementation of the LDI strategy will be phased in over time by using a glide path. The glide path is designed to increase the allocation of the plan’s assets to fixed income securities, as the funded status of the plan increases, in order to more closely match the duration of the plan assets to that of the plan liability. During the third quarter of 2016, qualifying term-vested participants were offered a lump-sum one-half In August 2016, Russell Investments Trust Company (“Russell”), an outside professional investment manager as defined in Section 3(38) of ERISA, was engaged as a fiduciary of the pension plan. Russell has full discretionary authority to direct the investment of the pension plan’s assets within the guidelines prescribed by the pension plan’s investment policy statement. The change, related to managing pension plan assets, has no impact on retirement benefit calculations for pension plan participants, and was approved by the Board of Directors of the Company. Pension plan assets are held in a Master Trust. Investment objectives and strategies for the qualified retirement plan are developed and approved by the Pension Plan Investment Committee of the Board of Directors. The objective of the investment policy is to manage assets in such a way that will allow the eventual settlement of the obligations to the pension plan’s beneficiaries. To meet this objective, the pension plan assets are managed by an outside adviser using a portfolio strategy that will provide liquidity to meet the plan’s benefit payment obligations. The pension plan funding policy is in compliance with the federal government’s funding requirements. Pension costs for these plans are affected by the amount and timing of cash contributions to the plans, the return on plan assets, discount rates, and by employee demographics, including age, compensation, and length of service. Changes made to the provisions of the plans may also impact current and future pension costs. Actuarial formulas are used in the determination of pension costs and are affected by actual plan experience and assumptions about future experience. Key actuarial assumptions include the expected return on plan assets, the discount rate used in determining the projected benefit obligation and pension costs, and the assumed rate of increase in employee compensation. Relatively small changes in these assumptions, particularly the discount rate, may significantly affect pension costs and plan obligations for the qualified retirement plan. In determining the discount rate, management matches the plan’s projected cash flows to a spot-rate yield curve based on highly rated corporate bonds. Changes to the discount rate from year-to-year, There was no change in the discount rate between years. The methodology utilized to determine the discount rate was consistent with prior years. The weighted-average rate of compensation increase also remained the same (consistent with management’s expectations overall). The asset return assumption (which impacts the following year’s expense) was lowered. The rates are presented in the table below: December 31, 2016 December 31, 2015 Discount rate 4.50 % 4.50 % Weighted-average rate of compensation increase 3.25 % 3.25 % Asset return assumption 7.00 % 7.25 % Pension expense for 2017 is estimated to be similar to that experienced in 2016. Future years’ expense level movements (up or down) will continue to be greatly influenced by long-term interest rates, asset returns, and funding levels. The following table sets forth the retirement plan, SERP, and PBOP funded statuses and amounts recognized on the Consolidated Balance Sheets and Consolidated Statements of Income. 2016 2015 Qualified Retirement Plan SERP PBOP Qualified Retirement Plan SERP PBOP (Thousands of dollars) Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO) $ 1,044,817 $ 42,720 $ 72,632 $ 1,060,240 $ 41,176 $ 72,202 Service cost 22,833 331 1,499 25,123 320 1,641 Interest cost 46,027 1,859 3,180 44,229 1,695 2,999 Actuarial loss (gain) 8,550 1,347 (2,060 ) (44,553 ) 2,322 (3,251 ) Benefits paid (73,874 ) (2,946 ) (1,386 ) (40,222 ) (2,793 ) (959 ) Benefit obligation at end of year (PBO/PBO/APBO) 1,048,353 43,311 73,865 1,044,817 42,720 72,632 Change in plan assets Market value of plan assets at beginning of year 736,880 — 43,584 754,796 — 44,892 Actual return on plan assets 39,956 — 4,818 (13,694 ) — (1,034 ) Employer contributions 36,000 2,946 — 36,000 2,793 — Benefits paid (73,874 ) (2,946 ) (289 ) (40,222 ) (2,793 ) (274 ) Market value of plan assets at end of year 738,962 — 48,113 736,880 — 43,584 Funded status at year end $ (309,391 ) $ (43,311 ) $ (25,752 ) $ (307,937 ) $ (42,720 ) $ (29,048 ) Weighted-average assumptions (benefit obligation) Discount rate 4.50 % 4.50 % 4.50 % 4.50 % 4.50 % 4.50 % Weighted-average rate of compensation increase 3.25 % 3.25 % N/A 3.25 % 3.25 % N/A Estimated funding for the plans above during calendar year 2017 is approximately $39 million, of which $36 million pertains to the retirement plan. Management monitors plan assets and liabilities and could, at its discretion, increase plan funding levels above the minimum in order to achieve a desired funded status and avoid or minimize potential benefit restrictions. The accumulated benefit obligation for the retirement plan and the SERP is presented below (in thousands): December 31, 2016 December 31, 2015 Retirement plan $ 939,002 $ 922,992 SERP 40,852 39,270 Benefits expected to be paid for pension, SERP, and PBOP over the next 10 years are as follows (in millions): 2017 2018 2019 2020 2021 2022-2026 Pension $ 48.6 $ 50.1 $ 51.5 $ 53.2 $ 55.1 $ 294.2 SERP 2.9 2.9 2.9 2.9 2.9 14.4 PBOP 4.1 4.3 4.4 4.5 4.5 20.6 No assurance can be made that actual funding and benefits paid will match these estimates. For PBOP measurement purposes, the per capita cost of the covered health care benefits medical rate trend assumption is 7% declining to 4.5%. Fixed contributions are made for health care benefits of employees who retire after 1988, but Southwest pays all covered health care costs for employees who retired prior to 1989. The medical trend rate assumption noted above applies to the benefit obligations of pre-1989 Components of net periodic benefit cost Qualified Retirement Plan SERP PBOP 2016 2015 2014 2016 2015 2014 2016 2015 2014 (Thousands of dollars) Service cost $ 22,833 $ 25,123 $ 21,360 $ 331 $ 320 $ 292 $ 1,499 $ 1,641 $ 1,101 Interest cost 46,027 44,229 43,440 1,859 1,695 1,745 3,180 2,999 2,829 Expected return on plan assets (56,558 ) (57,808 ) (53,342 ) — — — (3,149 ) (3,464 ) (3,264 ) Amortization of prior service cost — — — — — — 1,335 1,335 355 Amortization of net actuarial loss 25,266 32,743 22,873 1,383 1,293 783 417 345 — Net periodic benefit cost $ 37,568 $ 44,287 $ 34,331 $ 3,573 $ 3,308 $ 2,820 $ 3,282 $ 2,856 $ 1,021 Weighted-average assumptions (net benefit cost) Discount rate 4.50 % 4.25 % 5.00 % 4.50 % 4.25 % 5.00 % 4.50 % 4.25 % 5.00 % Expected return on plan assets 7.25 % 7.75 % 7.75 % N/A N/A N/A 7.25 % 7.75 % 7.75 % Weighted-average rate of compensation increase 3.25 % 2.75 % 3.25 % 3.25 % 2.75 % 3.25 % N/A N/A N/A Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income 2016 2015 2014 Total Qualified SERP PBOP Total Qualified SERP PBOP Total Qualified SERP PBOP (Thousands of dollars) Net actuarial loss (gain) (a) $ 22,770 $ 25,153 $ 1,347 $ (3,730 ) $ 30,519 $ 26,949 $ 2,322 $ 1,248 $ 173,646 $ 163,215 $ 5,460 $ 4,971 Amortization of prior service cost (b) (1,335 ) — — (1,335 ) (1,335 ) — — (1,335 ) (355 ) — — (355 ) Amortization of net actuarial loss (b) (27,066 ) (25,266 ) (1,383 ) (417 ) (34,381 ) (32,743 ) (1,293 ) (345 ) (23,656 ) (22,872 ) (784 ) — Prior service cost — — — — — — — — 6,661 — — 6,661 Regulatory adjustment 5,584 102 — 5,482 5,646 5,214 — 432 (140,308 ) (129,031 ) — (11,277 ) Recognized in other comprehensive (income) loss (47 ) (11 ) (36 ) — 449 (580 ) 1,029 — 15,988 11,312 4,676 — Net periodic benefit costs recognized in net income 44,423 37,568 3,573 3,282 50,451 44,287 3,308 2,856 38,172 34,331 2,820 1,021 Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss $ 44,376 $ 37,557 $ 3,537 $ 3,282 $ 50,900 $ 43,707 $ 4,337 $ 2,856 $ 54,160 $ 45,643 $ 7,496 $ 1,021 The table above discloses the net gain or loss and prior service cost recognized in other comprehensive income, separated into (a) amounts initially recognized in other comprehensive income, and (b) amounts subsequently recognized as adjustments to other comprehensive income as those amounts are amortized as components of net periodic benefit cost. See also Note 5 – Other Comprehensive Income and Accumulated Other Comprehensive Income (“AOCI”). U.S. GAAP states that a fair value measurement should be based on the assumptions that market participants would use in pricing the asset or liability and establishes a fair value hierarchy that ranks the inputs used to measure fair value by their reliability. The three levels of the fair value hierarchy are as follows: Level 1 – quoted prices (unadjusted) in active markets for identical assets or liabilities that a company has the ability to access at the measurement date. Level 2 – inputs other than quoted prices included within Level 1 that are observable for similar assets or liabilities, either directly or indirectly. Level 3 – unobservable inputs for the asset or liability. Unobservable inputs are used to measure fair value to the extent that observable inputs are not available, thereby allowing for situations in which there is little, if any, market activity for the asset or liability at the measurement date. The following table sets forth, by level within the three-level fair value hierarchy, the fair values of the assets of the qualified pension plan and the PBOP as of December 31, 2016 and December 31, 2015. The change in the types of pension investment holdings between years is due to the engagement of Russell and the subsequent transition of pension investments to Russell funds. The transition efforts consolidated the majority of the pension plan investments into private commingled equity and fixed income funds. The SERP has no assets. December 31, 2016 December 31, 2015 Qualified PBOP Total Qualified PBOP Total Assets at fair value (thousands of dollars): Level 1 – Quoted prices in active markets for identical financial assets Common stock Agriculture $ — $ — $ — $ 7,021 $ 209 $ 7,230 Capital equipment — — — 533 16 549 Chemicals/materials — — — 3,304 98 3,402 Consumer goods — — — 41,035 1,221 42,256 Energy and mining — — — 11,066 329 11,395 Finance/insurance — — — 29,957 892 30,849 Healthcare — — — 37,930 1,129 39,059 Information technology — — — 29,229 870 30,099 Services — — — 12,341 367 12,708 Telecommunications/internet/media — — — 25,883 770 26,653 Other — — — 9,043 269 9,312 Real estate investment trusts — — — 5,010 149 5,159 Mutual funds — 24,922 24,922 87,483 23,985 111,468 Government fixed income securities — — — 33,482 996 34,478 Futures contracts — — — (7 ) — (7 ) Total Level 1 Assets (1) $ — $ 24,922 $ 24,922 $ 333,310 $ 31,300 $ 364,610 Level 2 – Significant other observable inputs Private commingled equity funds (2) International $ 290,668 $ 9,140 $ 299,808 $ — $ — $ — Large and medium capitalization 121,434 3,819 125,253 — — — Small capitalization 25,947 816 26,763 — — — Emerging markets 45,309 1,424 46,733 — — — Private commingled fixed income funds (3) U.S. corporate bonds 161,086 5,066 166,152 — — — U.S. debt market long duration 77,349 2,432 79,781 — — — U.S. Treasury securities 8,665 272 8,937 — — — Pooled funds and mutual funds 4,889 216 5,105 14,808 796 15,604 Government fixed income and mortgage backed securities 167 5 172 49,571 1,475 51,046 Corporate fixed income securities Asset-backed and mortgage-backed — — — 23,542 701 24,243 Banking — — — 20,857 621 21,478 Insurance — — — 4,896 146 5,042 Utilities — — — 3,826 114 3,940 Other — — — 30,995 922 31,917 Real estate investment trusts — — — 1,949 58 2,007 State and local obligations — — — 950 28 978 Preferred securities — — — 554 17 571 Convertible securities — — — 196 6 202 Total Level 2 assets (4) $ 735,514 $ 23,190 $ 758,704 $ 152,144 $ 4,884 $ 157,028 Total Plan assets at fair value $ 735,514 $ 48,112 $ 783,626 $ 485,454 $ 36,184 $ 521,638 Commingled equity funds (5) — — — 250,511 7,455 257,966 Insurance company general account contracts (6) 3,448 — 3,448 3,719 — 3,719 Total Plan assets (7) $ 738,962 $ 48,112 $ 787,074 $ 739,684 $ 43,639 $ 783,323 (1) The Mutual funds category above is an intermediate-term bond fund whose manager employs multiple concurrent strategies and takes only moderate risk in each, thereby reducing the risk of poor performance arising from any single source, and a balanced fund that invests in a diversified portfolio of common stocks, preferred stocks and fixed-income securities. Strategies utilized by the bond fund include duration management, yield curve or maturity structuring, sector rotation, and all bottom-up in-house In the prior year, Level 1 also included Common stock, Real Estate Investment Trusts, Mutual funds, and U.S. Government securities listed or regularly traded on a national securities exchange and were valued at quoted market prices as of the last business day of the calendar year. (2) The private commingled equity funds include common collective trusts that invest in a diversified portfolio of domestic and international securities regularly traded on securities exchanges. These funds are shown in the above table at net asset value (“NAV”), which is the value of securities in the fund less the amount of any liabilities outstanding. Investment strategies employed by the funds include: • Domestic equities • International developed countries equities • Emerging markets equities Shares in the private equity commingled funds may be redeemed given one business day notice. While they are private equity funds and reported at NAV, due to the short redemption notice period, the lack of significant redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (level 2) and are also therefore, not excluded from the body of the fair value table as a reconciling item. Two funds are classified as international funds. One invests in international financial markets, primarily those of developed economies in Europe and the Pacific Basin. The fund invests primarily in equity securities issued by foreign corporations, but may invest in other securities perceived as offering attractive investment return opportunities. The other provides diversified exposure to global equity markets. The fund seeks to provide long-term capital growth by investing primarily in securities listed on the major developed equity markets of the United States, Europe, and Asia, as well as within those listed on emerging country equity markets on a tactical basis. The large and medium capitalization fund is designed to track the performance of the large and medium capitalization companies contained in the index, which represents approximately 90% of the market capitalization of the United States stock market. The small capitalization fund is designed to provide maximum long-term appreciation through investments that are well diversified by industry. The emerging markets fund was developed to invest in emerging market equities worldwide. The purposes of the fund’s operations, “emerging market countries” include every country in the world except the developed markets of the United States, Canada, Japan, Australia, New Zealand, Hong Kong and Singapore, and most countries located in Western Europe. Fund investments are made directly in each country or, where direct investment is inefficient or prohibited, through appropriate financial instruments or participation in commingled funds. (3) The private commingled fixed income funds include domestic fixed income securities. These funds are shown in the above table at NAV. Shares in the private commingled fixed equity funds may be redeemed given one business day notice. While they are private equity funds and reported at NAV, due to the short redemption notice period, the lack of significant redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (level 2) and are also therefore, not excluded from the body of the fair value table as a reconciling item. The U.S. corporate bond fund seeks to provide high quality, mostly corporate bond-based exposure to fixed income securities which closely match those found in discount curves used to value United States pension liabilities. The United States debt market long duration fund provides participation in the full spectrum of investment opportunities in primarily United States debt markets with longer maturities. The fund seeks to offer effective diversification against equities, take advantage of market trading opportunities, and provide a competitive rate of return on assets. The fund’s current duration is close to 14 years. The United States Treasuries securities funds seeks to replicate the risk and return characteristics of the Barclays Treasury U.S. Separate Trading of Registered Interest and Principal of Securities (“STRIPS”) 28-29 (4) With the exception of items (2) and (3), which are discussed in detail above, the current year Level 2 assets consist mainly of pooled funds and mutual funds. These funds are collective short-term funds that invest in Treasury bills and money market funds and are used as a temporary cash repository. In the prior year, the fair value of the Level 2 investments in debt securities with remaining maturities of one year or more was determined by dealers who make markets in such securities or by an independent pricing service, which considers yield or price of bonds of comparable quality, coupon, maturity, and type. (5) In the prior year, the commingled equity funds included private equity funds that invest in domestic and international securities regularly traded on securities exchanges. These funds are shown in the above table at net asset value, which is the value of securities in the fund less the amount of any liabilities outstanding. Investment strategies employed by the funds included: • Domestic large capitalization value equities • International developed countries value and growth equities • Emerging markets equities • International small capitalization equities The terms and conditions under which shares in the commingled equity funds were redeemed varied among the funds; the notice required ranged from one day to 30 days prior to the valuation date (month end). One of the commingled equity funds required the payment of a minimal impact fee to be applied to redemptions and subscriptions of $5 million or greater; the relative fee diminished the greater the transaction. Other such funds imposed fees to recover direct costs incurred by the fund at redemption, but were indeterminable prior to redemption. (6) The insurance company general account contracts are annuity insurance contracts used to pay the pensions of employees who retired prior to 1989. The balance of the account disclosed in the above table is the contract value, which is the result of deposits, withdrawals, and interest credits. (7) In the prior year, the assets in the above table exceeded the market value of plan assets shown in the funded status table by $2,859,000 (qualified retirement plan – $2,803,000, PBOP – $56,000), which includes a payable for securities purchased, partially offset by receivables for interest, dividends, and securities sold.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-Based Compensation	Note 11 – Stock-Based Compensation At December 31, 2016, two stock-based compensation plans existed: a performance share stock plan which includes a cash award, and a restricted stock/unit plan. All previous grants under the stock option plan expired in 2016. The table below shows total stock-based plan compensation expense, including the cash award, which was recognized in the Consolidated Statements of Income (in thousands): 2016 2015 2014 Stock-based compensation plan expense, net of related tax benefits $ 7,185 $ 7,278 $ 8,130 Stock-based compensation plan related tax benefits 4,404 4,461 4,983 Under the option plan, options to purchase shares of common stock at a stated exercise price were previously granted to key employees and outside directors. The last option grants were in 2006 and no future grants are anticipated. Each option had an exercise price equal to the market price of the Company’s common stock on the date of grant and a maximum term of ten years. The final options were exercised in 2016. The following tables summarize the stock option plan activity and related information (thousands of options): 2016 2015 2014 Number Weighted- Number of Weighted- Number of Weighted- Outstanding at the beginning of the year 17 $ 31.64 36 $ 28.97 52 $ 27.57 Exercised during the year (17 ) 31.64 (19 ) 26.69 (16 ) 24.31 Forfeited or expired during the year — — — — — — Outstanding and exercisable at year end — N/A 17 $ 31.64 36 $ 28.97 The intrinsic value of a stock option is the amount by which the market value of the underlying stock exceeds the exercise price of the option. The aggregate intrinsic value of outstanding and exercisable options, and options that were exercised, are presented in the table below (in thousands): 2016 2015 2014 Outstanding and exercisable $ — $ 394 $ 1,194 Exercised 554 590 451 December 31, 2016 December 31, 2015 December 31, 2014 Market value of Company stock $ 76.62 $ 55.16 $ 61.81 During 2016, $735,000 in cash was received from the exercise of options with a corresponding tax benefit of $205,000, which was recorded in additional paid-in Under the performance share stock plan, performance shares may be issued to encourage key employees to remain as employees and to achieve short-term and long-term performance goals. Plan participants are eligible to receive a cash bonus (i.e., short-term incentive) and performance shares (i.e., long-term incentive). The performance shares vest three years after grant and are then issued as common stock. Restricted stock/units under the restricted stock/unit plan are issued to attract, motivate, retain, and reward key employees with an incentive to attain high levels of individual performance and improved financial performance. The restricted stock/units vest 40% at the end of year one and 30% at the end of years two and three and are issued annually as common stock in accordance with the percentage vested. The restricted stock/unit plan was also established to attract, motivate, and retain experienced and knowledgeable independent directors. Vesting for grants of restricted stock/units to directors occurs immediately upon grant. The issuance of common stock for directors currently occurs when their service on the Board ends. The following table summarizes the activity of the performance share stock and restricted stock/unit plans as of December 31, 2016 (thousands of shares): Performance Weighted- Restricted Weighted- Nonvested/unissued at beginning of year 197 $ 50.63 228 $ 44.36 Granted 44 59.05 73 60.39 Dividends 5 6 Forfeited or expired — — — — Vested and issued* (78 ) 41.82 (45 ) 51.98 Nonvested/unissued at December 31, 2016 168 $ 55.62 262 $ 46.41 * Includes shares for retiree payouts and those converted for taxes. The weighted average grant date fair value of performance shares and restricted stock/units granted in 2015 and 2014 was $63.09 and $53.73, respectively. As of December 31, 2016, total compensation cost related to nonvested performance shares and restricted stock/units not yet recognized is $3.3 million.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income Taxes	Note 12 – Income Taxes The following is a summary of income before taxes and noncontrolling interest for domestic and foreign operations (thousands of dollars): Year ended December 31, 2016 2015 2014 U.S. $ 218,810 $ 221,660 $ 221,471 Foreign 12,713 (2,328 ) (1,950 ) Total income before income taxes $ 231,523 $ 219,332 $ 219,521 Income tax expense (benefit) consists of the following (thousands of dollars): Year Ended December 31, 2016 2015 2014 Current: Federal $ 541 $ 21,321 $ 1,739 State 5,748 9,899 5,073 Foreign 4,298 650 2,193 10,587 31,870 9,005 Deferred: Federal 68,270 51,132 71,439 State 140 (2,574 ) 614 Foreign (529 ) (526 ) (2,685 ) 67,881 48,032 69,368 Total income tax expense $ 78,468 $ 79,902 $ 78,373 Deferred income tax expense (benefit) consists of the following significant components (thousands of dollars): Year Ended December 31, 2016 2015 2014 Deferred federal and state: Property-related items $ 76,217 $ 65,931 $ 52,814 Purchased gas cost adjustments 361 (32,993 ) 15,049 Employee benefits (1,327 ) 623 109 All other deferred (6,532 ) 15,332 2,257 Total deferred federal and state 68,719 48,893 70,229 Deferred ITC, net (838 ) (861 ) (861 ) Total deferred income tax expense $ 67,881 $ 48,032 $ 69,368 A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate for 2014, 2015, and 2016 (and the sources of these differences and the effect of each) are summarized as follows: Year Ended December 31, 2016 2015 2014 U.S. federal statutory income tax rate 35.0 % 35.0 % 35.0 % Net state taxes 1.4 1.8 1.9 Property-related items — 0.1 0.1 Tax credits (0.4 ) (0.4 ) (0.5 ) Company owned life insurance (1.2 ) 0.1 (1.0 ) All other differences (0.9 ) (0.2 ) 0.2 Consolidated effective income tax rate 33.9 % 36.4 % 35.7 % Deferred tax assets and liabilities consist of the following (thousands of dollars): December 31, 2016 2015 Deferred tax assets: Deferred income taxes for future amortization of ITC $ 1,094 $ 1,614 Employee benefits 38,231 36,923 Alternative minimum tax credit 4,827 4,809 Net operating losses and credits 1,204 868 Interest rate swap 6,080 7,351 Other 18,415 24,636 Valuation allowance (495 ) (499 ) 69,356 75,702 Deferred tax liabilities: Property-related items, including accelerated depreciation 872,136 794,850 Regulatory balancing accounts 1,104 743 Unamortized ITC 1,710 2,549 Debt-related costs 5,712 5,497 Intangibles 8,803 9,547 Other 19,256 31,533 908,721 844,719 Net noncurrent deferred tax liabilities $ 839,365 $ 769,017 The Company and its subsidiaries file income tax returns in the U.S. federal jurisdiction, various states, and in Canada. With few exceptions, the Company is no longer subject to United States federal, state and local, or Canadian income tax examinations for years before 2012. At December 31, 2016, the Company has U.S. federal net capital loss carryforwards of $278,000, which begin to expire in 2017. At December 31, 2016, the Company has an income tax net operating loss carryforward related to Canadian operations of $4.5 million which begins to expire in 2032. As of December 31, 2016, the Company has approximately $5 million of undistributed foreign earnings. However, management intends to permanently reinvest any future foreign earnings in Canada. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (thousands of dollars): 2016 2015 Unrecognized tax benefits at beginning of year $ 296 $ 305 Gross increases – tax positions in prior period 897 — Gross decreases – tax positions in prior period — (9 ) Gross increases – current period tax positions 38 — Gross decreases – current period tax positions — — Settlements — — Lapse in statute of limitations — — Unrecognized tax benefits at end of year $ 1,231 $ 296 In assessing whether uncertain tax positions should be recognized in its financial statements, management first determines whether it is more-likely-than-not more-likely-than-not more-likely-than-not more-likely-than-not The total amount of unrecognized tax benefits that, if recognized, would impact the effective tax rate was $935,000 at December 31, 2016. No significant increases or decreases in unrecognized tax benefit are expected within the next 12 months. The Company recognizes interest expense and income and penalties related to income tax matters in income tax expense. There was no tax-related Income Tax Regulations

Derivatives and Fair Value Meas

Derivatives and Fair Value Measurements	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Derivatives and Fair Value Measurements	Note 13 – Derivatives and Fair Value Measurements Derivatives. fixed-for-floating The fixed-price contracts and Swaps are utilized by Southwest under its volatility mitigation programs to effectively fix the price on a portion (up to 25% in the Arizona and California jurisdictions) of its natural gas supply portfolios. The maturities of the Swaps highly correlate to forecasted purchases of natural gas, during time frames ranging from January 2017 through March 2019. Under such contracts, Southwest pays the counterparty a fixed rate and receives from the counterparty a floating rate per MMBtu (“dekatherm”) of natural gas. Only the net differential is actually paid or received. The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms): December 31, 2016 December 31, 2015 Contract notional amounts 10,543 7,407 Southwest does not utilize derivative financial instruments for speculative purposes, nor does it have trading operations. The following table sets forth the gains and (losses) recognized on Southwest’s Swaps (derivatives) for the years ended December 31, 2016, 2015, and 2014 and their location in the Consolidated Statements of Income: Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Instrument Location of Gain or (Loss) 2016 2015 2014 Swaps Net cost of gas sold $ 5,006 $ (7,598 ) $ (2,363 ) Swaps Net cost of gas sold (5,006 )* 7,598 * 2,363 * Total $ — $ — $ — * Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities. No gains (losses) were recognized in net income or other comprehensive income during the periods presented for derivatives designated as cash flow hedging instruments. Previously, Southwest entered into two forward-starting interest rate swaps (“FSIRS”), both of which were designated cash flow hedges, to partially hedge the risk of interest rate variability during the period leading up to the planned issuance of debt. The first FSIRS terminated in December 2010, and the second, in March 2012. Losses on both FSIRS are being amortized over ten-year The following table sets forth the fair values of the Swaps and their location in the Consolidated Balance Sheets (thousands of dollars): Fair values of derivatives not designated as hedging instruments: December 31, 2016 Instrument Balance Sheet Location Asset Liability Net Swaps Deferred charges and other assets $ 899 $ (54 ) $ 845 Swaps Prepaids and other current assets 3,551 (19 ) 3,532 Total $ 4,450 $ (73 ) $ 4,377 December 31, 2015 Instrument Balance Sheet Location Asset Liability Net Swaps Other current liabilities $ — $ (4,267 ) $ (4,267 ) Swaps Other deferred credits 4 (1,223 ) (1,219 ) Total $ 4 $ (5,490 ) $ (5,486 ) The estimated fair values of the natural gas derivatives were determined using future natural gas index prices (as more fully described below). Master netting arrangements exist with each counterparty that provide for the net settlement (in the settlement month) of all contracts through a single payment. As applicable, management has elected to reflect the net amounts in its balance sheets. No outstanding collateral associated with the Swaps existed during any period presented in the above table. Pursuant to regulatory deferral accounting treatment for rate-regulated entities, unrealized gains and losses in fair value of the Swaps are recorded as a regulatory asset and/or liability. When the Swaps mature, any prior positions held are reversed and the settled position is recorded as an increase or decrease of purchased gas under the related purchased gas adjustment (“PGA”) mechanism in determining its deferred PGA balances. Neither changes in fair value, nor settled amounts, of Swaps have a direct effect on earnings or other comprehensive income. The following table presents the amounts paid to and received from counterparties for settlements of matured Swaps. Year ended Year ended Year ended (Thousands of dollars) Paid to counterparties $ 5,583 $ 7,537 $ 829 Received from counterparties $ 726 $ — $ 4,713 The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Consolidated Balance Sheets (thousands of dollars). December 31, 2016 Instrument Balance Sheet Location Net Total Swaps Other deferred credits $ (845 ) Swaps Other current liabilities (3,532 ) December 31, 2015 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 4,267 Swaps Deferred charges and other assets 1,219 Fair Value Measurements. The following table sets forth, by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, financial assets and liabilities that were accounted for at fair value (see Note 10 – Pension and Other Post Retirement Benefits for definitions of the levels of the fair value hierarchy): Level 2 – Significant other observable inputs December 31, 2016 December 31, 2015 (Thousands of dollars) Assets at fair value: Prepaids and other current assets – Swaps $ 3,532 $ — Deferred charges and other assets – Swaps 845 — Liabilities at fair value: Other current liabilities – Swaps — (4,267 ) Other deferred credits – Swaps — (1,219 ) Net Assets (Liabilities) $ 4,377 $ (5,486 ) No financial assets or liabilities associated with the Swaps, which were accounted for at fair value, fell within Level 1 or Level 3 of the fair value hierarchy. With regard to the fair values of assets associated with pension and postretirement benefit plans, refer to Note 10 – Pension and Other Post Retirement Benefits .

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2016
Segment Reporting [Abstract]
Segment Information	Note 14 – Segment Information Operating segments are determined based on the nature of their activities. The natural gas operations segment is engaged in the business of purchasing, distributing, and transporting natural gas. Revenues are generated from the distribution and transportation of natural gas. The construction services segment is primarily engaged in the business of providing utility companies with trenching and installation, replacement, and maintenance services for energy distribution systems, and providing industrial construction solutions. Over 99% of the total Company’s long-lived assets are in the United States. The accounting policies of the reported segments are the same as those described within Note 1 – Summary of Significant Accounting Policies . Centuri accounts for the services provided to Southwest at contractual prices at contract inception. Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below (in thousands). December 31, 2016 December 31, 2015 Accounts receivable for Centuri services $ 10,585 $ 10,006 The following table presents the amount of revenues for both segments by geographic area (thousands of dollars): December 31, December 31, December 31, Revenues (a) United States $ 2,256,600 $ 2,289,133 $ 2,069,513 Canada 203,890 174,492 52,194 Total $ 2,460,490 $ 2,463,625 $ 2,121,707 (a) Revenues are attributed to countries based on the location of customers. The financial information pertaining to the natural gas operations and construction services segments for each of the three years in the period ended December 31, 2016 is as follows (thousands of dollars): 2016 Gas Construction Adjustments Total Revenues from unaffiliated customers $ 1,321,412 $ 1,040,957 $ 2,362,369 Intersegment sales — 98,121 98,121 Total $ 1,321,412 $ 1,139,078 $ 2,460,490 Interest revenue $ 1,848 $ 1 $ 1,849 Interest expense $ 66,997 $ 6,663 $ 73,660 Depreciation and amortization $ 233,463 $ 55,669 $ 289,132 Income tax expense $ 58,584 $ 19,884 $ 78,468 Segment net income $ 119,423 $ 32,618 $ 152,041 Segment assets $ 5,001,756 $ 579,370 $ 5,581,126 Capital expenditures $ 457,120 $ 72,411 $ 529,531 2015 Gas Construction Adjustments Total Revenues from unaffiliated customers $ 1,454,639 $ 904,870 $ 2,359,509 Intersegment sales — 104,116 104,116 Total $ 1,454,639 $ 1,008,986 $ 2,463,625 Interest revenue $ 1,754 $ 419 $ 2,173 Interest expense $ 64,095 $ 7,784 $ 71,879 Depreciation and amortization $ 213,455 $ 56,656 $ 270,111 Income tax expense $ 61,355 $ 18,547 $ 79,902 Segment net income $ 111,625 $ 26,692 $ 138,317 Segment assets $ 4,822,845 $ 535,840 $ 5,358,685 Capital expenditures $ 438,289 $ 49,711 $ 488,000 2014 Gas Construction Adjustments (a) Total Revenues from unaffiliated customers $ 1,382,087 $ 647,432 $ 2,029,519 Intersegment sales — 92,188 92,188 Total $ 1,382,087 $ 739,620 $ 2,121,707 Interest revenue $ 2,596 $ 6 $ 2,602 Interest expense $ 68,299 $ 3,770 $ 72,069 Depreciation and amortization $ 204,144 $ 48,883 $ 253,027 Income tax expense $ 63,597 $ 14,776 $ 78,373 Segment net income $ 116,872 $ 24,254 $ 141,126 Segment assets $ 4,652,307 $ 566,589 $ (10,599 ) $ 5,208,297 Capital expenditures $ 350,025 $ 46,873 $ 396,898 (a) Construction services segment assets included two liabilities that were netted against gas operations segment assets during consolidation in 2014. They are: Income taxes payable of $3.3 million, netted against income taxes receivable, net and deferred income taxes of $1.4 million, netted against deferred income taxes, net. Construction services segment assets exclude a long-term deferred tax benefit of $1.4 million, which was netted against gas operations segment deferred income taxes and investment tax credits, net during consolidation. Gas operations segment assets include a deferred income tax liability of $4.5 million, which was netted against a construction services segment asset for deferred income taxes, net during consolidation.

Quarterly Financial Data

Quarterly Financial Data	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Data	Note 15 – Quarterly Financial Data (Unaudited) Quarter Ended March 31 June 30 September 30 December 31 (Thousands of dollars, except per share amounts) 2016 Operating revenues $ 731,248 $ 547,748 $ 539,969 $ 641,525 Operating income 134,096 28,116 15,539 117,963 Net income 75,355 9,099 2,907 65,694 Net income attributable to Southwest Gas Corporation 75,446 8,943 2,472 65,180 Basic earnings per common share* 1.59 0.19 0.05 1.37 Diluted earnings per common share* 1.58 0.19 0.05 1.36 2015 Operating revenues $ 734,220 $ 538,604 $ 505,396 $ 685,405 Operating income 129,556 25,047 16,143 117,586 Net income (loss) 71,879 5,063 (4,210 ) 66,698 Net income (loss) attributable to Southwest Gas Corporation 71,983 4,949 (4,734 ) 66,119 Basic earnings (loss) per common share* 1.54 0.11 (0.10 ) 1.40 Diluted earnings (loss) per common share* 1.53 0.10 (0.10 ) 1.38 2014 Operating revenues $ 608,396 $ 453,153 $ 432,475 $ 627,683 Operating income 127,065 26,755 18,290 112,373 Net income 70,697 9,627 1,927 58,897 Net income attributable to Southwest Gas Corporation 70,783 9,627 1,970 58,746 Basic earnings per common share* 1.52 0.21 0.04 1.26 Diluted earnings per common share* 1.51 0.21 0.04 1.25 * The sum of quarterly earnings (loss) per average common share may not equal the annual earnings (loss) per share due to the ongoing change in the weighted-average number of common shares outstanding. The demand for natural gas is seasonal, and it is the opinion of management that comparisons of earnings for interim periods do not reliably reflect overall trends and changes in operations. Also, the timing of general rate relief can have a significant impact on earnings for interim periods. See Management’s Discussion and Analysis for additional discussion of operating results.

Construction Services Noncontro

Construction Services Noncontrolling Interests	12 Months Ended
	Dec. 31, 2016
Noncontrolling Interest [Abstract]
Construction Services Noncontrolling Interests	Note 16 – Construction Services Noncontrolling Interests Associated with the agreement reached in conjunction with the acquisition of the Canadian construction businesses in October 2014, the previous owners of the acquired companies initially retained an approximate 10% equity interest in the Canadian-specific businesses, and special dividend rights which entitled the sellers, as holders, to dividends equal to 3.4% of dividends paid at the level of Centuri, and subject to certain conditions, such interests could become exchangeable for a 3.4% equity interest in Centuri. In consideration of the underlying exchange rights of the original agreement, earnings attribution by Centuri to the previous owners also occurred in an amount equivalent to 3.4% of Centuri earnings since October 2014. During the third quarter of 2015, the sellers formally exercised their exchange rights under the terms of the original agreement. No new rights were conveyed to the noncontrolling parties as a result of the exchange and no new consideration was involved. The previous owners are currently eligible to exit their investment retained by requiring the purchase of a portion of their interest and in incremental amounts annually. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election beginning in July 2022. Due to the ability of the noncontrolling parties to redeem their interest in Centuri for cash, their collective interest is presented on the Consolidated Balance Sheets at December 31, 2016 and December 31, 2015 as a Redeemable noncontrolling interest, a category of mezzanine equity (temporary equity), in accordance with SEC guidance. Significant changes in the value of the redeemable noncontrolling interest are recognized as they occur, and the carrying value is adjusted as necessary at each reporting date. Guidance by the SEC indicates that downward adjustments in the value of redeemable noncontrolling interests are only permitted to the extent that upward adjustments in value were previously recognized. A floor for the noncontrolling interest was originally set at the acquisition date (in October 2014). However, U.S. GAAP generally views changes in ownership interest, where the parent retains its controlling interest, as an equity transaction, whereby the carrying amount of the noncontrolling interest is adjusted to reflect the change in ownership interest in the subsidiary. In connection with the exchange rights exercised during the third quarter of 2015, an updated valuation was conducted. A significant decrease in the value of the redeemable noncontrolling interest was recognized at that time, due in part to the exchange option no longer being subject to probability estimates. In light of the U.S. GAAP requirement to adjust the carrying amount, a new floor was set for the redeemable noncontrolling interest at the exchange date (July 31, 2015), with a corresponding adjustment made to additional paid-in Redeemable (Thousands of dollars): Balance, December 31, 2015 $ 16,108 Net Income (loss) attributable to redeemable noncontrolling interest 1,148 Foreign currency exchange translation adjustment 5 Centuri distribution to redeemable noncontrolling interest (439 ) Adjustment to redemption value 5,768 Balance, December 31, 2016 $ 22,590 The redemption value of the redeemable noncontrolling interest utilizes a market approach to determine a construction services enterprise value. Publicly traded “guideline” companies are identified by using a selection criteria, including actively traded equities, their financial solvency, and other factors. Once the guideline companies are determined, enterprise value is calculated using a weighted approach of projected earnings before interest expense and taxes (“EBIT”) and earnings before interest expense, taxes, and depreciation and amortization expense (“EBITDA”). After an estimated fair value is determined, it is multiplied by 3.4%. A discount is then applied due to limitations of the nonpublic noncontrolling interest being valued. Each quarter, market changes in the guideline companies are considered and the weighted approach to projected EBIT and EBITDA, in relation to the guideline companies, is re-evaluated Centuri also holds a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning (“HVAC”) technology and products. Centuri consolidates the entity (IntelliChoice Energy, LLC) as a majority-owned subsidiary. The interest is immaterial to the consolidated financial statements, but is identified as the Noncontrolling interest within Total equity on the Consolidated Balance Sheets.

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Holding Company Reorganization	Holding Company Reorganization. one-for-one
Nature of Operations	Nature of Operations. Acquisition of Construction Services Businesses
Basis of Presentation	Basis of Presentation.
Consolidation	Consolidation. Centuri, through its subsidiaries, holds a 65% interest in a venture to market natural gas engine-driven heating, ventilating, and air conditioning (“HVAC”) technology and products. Centuri consolidates the entity (IntelliChoice Energy, LLC). Centuri, through its subsidiaries, holds a 50% interest in W.S. Nicholls Western Construction LTD. (“Western”), a Canadian construction services company that is a variable interest entity. Centuri determined that it is not the primary beneficiary of the entity due to a shared-power structure; therefore, Centuri does not consolidate the entity and has recorded its investment, and results related thereto, using the equity method. The investment in Western totaled $10.8 million at December 31, 2015 and 2016. Both periods include the impacts of foreign currency exchange translation adjustments. Dividends of $500,000 were received from Western during 2016 with no impact on earnings. No dividends were received from Western in 2015. A management fee was paid by Western to its partners, including W.S. Nicholls, in accordance with underlying agreements. The equity method investment in Western is included in Other Property and Investments in the Consolidated Balance Sheets. Centuri’s maximum exposure to loss as a result of its involvement with Western is estimated at $35.8 million. The estimated maximum exposure to loss represents the maximum loss that would be absorbed by Centuri in the event that all of the assets of Western were deemed to be worthless. Centuri recorded earnings of $69,000 from this investment in 2016, which is included in Other Income (deductions) in the Consolidated Statements of Income. In addition, Centuri, through its subsidiaries, has a 25% interest in CCI-TBN
Net Utility Plant	Net Utility Plant.
Other Property and Investments	Other Property and Investments. 2016 2015 Centuri property, equipment, and intangibles $ 451,114 $ 423,369 Centuri accumulated provision for depreciation and amortization (228,374 ) (221,028 ) Net cash surrender value of COLI policies 106,744 99,276 Other property 12,859 11,914 Total $ 342,343 $ 313,531
Deferred Purchased Gas Costs	Deferred Purchased Gas Costs.
Prepaids and Other Current Assets	Prepaids and other current assets
Income Taxes	Income Taxes.
Cash and Cash Equivalents	Cash and Cash Equivalents. Significant non-cash non-cash non-cash non-cash
Goodwill	Goodwill. th Natural Construction Consolidated (In thousands of dollars) December 31, 2015 $ 10,095 $ 116,050 $ 126,145 Additional goodwill from ETTI acquisition — 10,726 10,726 Foreign currency translation adjustment — 3,112 3,112 December 31, 2016 $ 10,095 $ 129,888 $ 139,983 Goodwill from the ETTI acquisition consists of the excess of purchase price over the fair value of the acquired net assets and represents the value of the assembled workforce and the estimated economic value attributable to future opportunities that will arise based on the strong financial performance of the combined entities.
Acquisition of Construction Services Businesses	Acquisition of Construction Services Businesses. Assets acquired in the transaction were recorded at their acquisition date fair values. The final purchase accounting is complete. The final estimated fair values of assets acquired as of May 6, 2016, the acquisition date, are as follows (in millions of dollars): Acquisition Property, plant and equipment $ 4.3 Intangible assets 2.9 Goodwill 10.7 Total assets acquired $ 17.9 The purchase price consisted of $17 million in cash on the acquisition date with the remaining amount being deferred over four years.
Intangible Assets	Intangible Assets December 31, 2016 Gross Carrying Accumulated Net Carrying Customer relationships $ 34,033 $ (3,906 ) $ 30,127 Trade names and trademarks 9,349 (2,565 ) 6,784 Customer contracts backlog 1,656 (1,656 ) — Noncompete agreement 1,029 (271 ) 758 Total $ 46,067 $ (8,398 ) $ 37,669 December 31, 2015 Customer relationships $ 31,226 $ (2,070 ) $ 29,156 Trade names and trademarks 8,621 (1,331 ) 7,290 Customer contracts backlog 1,606 (1,606 ) — Noncompete agreement 437 (110 ) 327 Total $ 41,890 $ (5,117 ) $ 36,773 The intangible assets (other than goodwill and software-related intangibles) are included in Other property and investments in the Consolidated Balance Sheets. The estimated future amortization of the intangible assets for the next five years is as follows (in thousands): 2017 $ 3,339 2018 3,126 2019 2,463 2020 2,395 2021 2,269 See Note 2 – Utility Plant and Leases for additional information regarding natural gas operations intangible assets.
Accumulated Removal Costs	Accumulated Removal Costs.
Gas Operating Revenues	Gas Operating Revenues. The Company acts as an agent for state and local taxing authorities in the collection and remission of a variety of taxes, including sales and use taxes and surcharges. These taxes are not included in gas operating revenues. Management uses the net classification method to report taxes collected from customers to be remitted to governmental authorities.
Construction Revenues	Construction Revenues. percentage-of-completion
Construction Expenses	Construction Expenses. job-related
Net Cost of Gas Sold	Net Cost of Gas Sold.
Operations and Maintenance Expense	Operations and Maintenance Expense.
Depreciation and Amortization	Depreciation and Amortization. Accumulated Removal Costs
Allowance for Funds Used During Construction ("AFUDC")	Allowance for Funds Used During Construction (“AFUDC”). 2016 2015 2014 (In thousands) AFUDC: Debt portion $ 1,175 $ 1,666 $ 1,228 Equity portion 2,289 3,008 1,995 AFUDC capitalized as part of utility plant $ 3,464 $ 4,674 $ 3,223 AFUDC rate 7.35 % 7.32 % 7.73 %
Other Income (Deductions)	Other Income (Deductions). 2016 2015 2014 Change in COLI policies $ 7,400 $ (500 ) $ 5,300 Interest income 1,849 2,173 2,602 Equity AFUDC 2,289 3,008 1,995 Foreign currency transaction gain (loss) (22 ) (824 ) (178 ) Equity in earnings of unconsolidated investment - Western 69 310 107 Miscellaneous income and (expense) (2,116 ) (1,288 ) (2,719 ) Total other income (deductions) $ 9,469 $ 2,879 $ 7,107 Included in the table above is the change in cash surrender values of company-owned life insurance (“COLI”) policies (including net death benefits recognized). These life insurance policies on members of management and other key employees are used by the Company to indemnify itself against the loss of talent, expertise, and knowledge, as well as to provide indirect funding for certain nonqualified benefit plans. Current tax regulations provide for tax-free
Foreign Currency Translation	Foreign Currency Translation.
Earnings Per Share	Earnings Per Share. 2016 2015 2014 (In thousands) Average basic shares 47,469 46,992 46,494 Effect of dilutive securities: Stock options 1 8 17 Performance shares 124 171 215 Restricted stock units 220 212 218 Average diluted shares 47,814 47,383 46,944
Recently Issued Accounting Standards Updates	Recently Issued Accounting Standards Updates. one-year Management has substantially completed the evaluation of the sources of revenue and are currently assessing the effect of the new guidance on the financial position, results of operations and cash flows. The assessment is contingent, in part, upon the completion of deliberations currently in progress by the utility industry, notably in connection with efforts to produce an accounting guide intended to be developed by the American Institute of Certified Public Accountants (“AICPA”). In association with this undertaking, the AICPA formed a number of industry task forces, including a Power & Utilities (“P&U”) Task Force, on which Company personnel actively participate via formal membership. Industry representatives and organizations, the largest auditing firms, the AICPA’s Revenue Recognition Working Group and its Financial Reporting Executive Committee have undertaken, and continue to undertake, consideration of several items relevant to the utility industry. Where applicable or necessary, the FASB’s Transition Resource Group (TRG) is also participating. Currently, the industry is working to address several items including the evaluation of collectability from customers if a utility has regulatory mechanisms to help assure recovery of uncollected accounts from ratepayers and the accounting for funds received from third parties to partially or fully reimburse the cost of construction of an asset. Currently, a timeline for the resolution of these deliberations has not been established. Southwest is actively working with its peers in the rate-regulated natural gas industry and with the public accounting profession to conclude on the accounting treatment for several other issues that are not expected to be addressed by the P&U Task Force. As of December 31, 2016, the construction services segment has substantially completed the evaluation of sources of revenue and is currently assessing the effect of the new guidance on financial position, results of operations and cash flows. The principals of the new revenue recognition guidance are very similar to existing guidance for construction contractors. Similar to the P&U Task Force noted above, the AICPA formed the Engineering and Construction Contractors Task Force to assist the construction industry with implementing the new guidance. The accounting guide the AICPA intends to release is expected to provide implementation guidance related to several issues including 1) combining contracts and separating performance obligations; 2) estimating change orders, incentives, penalties, liquidated damages and other variable consideration items and 3) acceptable measures of progress when recognizing revenue over time. Given the uncertainty with respect to the conclusions that might arise from the deliberations on issues associated with both the natural gas and construction services segments, the Company is currently unable to determine the effect the new guidance will have on its financial position, results of operations, cash flows, business processes, or the transition method it will utilize to adopt the new guidance. In January 2016, the FASB issued the update “Financial Instruments – Overall (Subtopic 825-10): In February 2016, the FASB issued the update “Leases (Topic 842)”. Under the update, lessees will be required to recognize the following for all leases (with the exception of short-term leases) at the commencement date: • A lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and • A right-of-use Under the new guidance, lessor accounting is largely unchanged. Certain targeted improvements were made to align, where necessary, lessor accounting with the lessee accounting model and Topic 606, Revenue from Contracts with Customers. Though companies have historically been required to make disclosures regarding leases and of contractual obligations, leases (with terms longer than a year) will no longer exist off-balance In March 2016, the FASB issued the update “Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting”. The amendments are intended to improve the accounting for employee share-based payments and affect all organizations that issue share-based payment awards to their employees. The update requires the recording of all of the tax effects related to share-based payments at settlement (or expiration) through the income statement. Currently, tax benefits in excess of compensation cost (“windfalls”) are recorded in equity, and tax deficiencies (“shortfalls”) are recorded in equity to the extent of previous windfalls, and then recorded in the income statement. While the simplification will reduce some of the administrative complexities by eliminating the need to track a “windfall pool,” it will increase the volatility of income tax expense. The update also allows entities to withhold shares for the employee tax burden up to the employees’ maximum individual tax rate in the relevant jurisdiction without resulting in a liability classification of the award (currently such withholding is limited to the employer’s minimum statutory withholding). The update clarifies that all cash payments made to taxing authorities on the employees’ behalf for withheld shares should be presented as financing activities on the statement of cash flows. Also, the update requires all tax-related In June 2016, the FASB issued the update “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments”. The update amends guidance on reporting credit losses for financial assets held at amortized cost basis and available for sale debt securities. For assets held at amortized cost basis, the update eliminates the “probable” threshold for initial recognition of credit losses in current U.S. GAAP and, instead, requires an entity to reflect its current estimate of all expected credit losses. The allowance for credit losses is a valuation account that is deducted from the amortized cost basis of the financial asset to present the net amount expected to be collected. For available for sale debt securities, credit losses should be measured in a manner similar to current U.S. GAAP, however the update will require that credit losses be presented as an allowance rather than as a write-down. This update affects entities holding financial assets and net investment in leases that are not accounted for at fair value through net income. The update affects loans, debt securities, trade receivables, net investments in leases, off-balance In August 2016, the FASB issued the update “Classification of Certain Cash Receipts and Cash Payments”. This update addresses the following specific cash flow issues: debt prepayment or debt extinguishment costs; settlement of zero-coupon In October 2016, the FASB issued the update “Accounting for Income Taxes: Intra-Entity Asset Transfers of Assets Other than Inventory.” This update eliminates the current U.S. GAAP exception for all intra-entity sales of assets other than inventory. As a result, a reporting entity would recognize the tax expense from the sale of the asset in the seller’s tax jurisdiction when the transfer occurs, even though the pre-tax In October 2016, the FASB issued the update “Consolidation (Topic 810): Interests Held through Related Parties That Are under Common Control.” The amendments affect reporting entities that are required to evaluate whether they should consolidate a variable interest entity in certain situations involving entities under common control. The update is effective for fiscal and interim periods beginning after December 15, 2016. Management has determined that this update is not impactful to its consolidated financial statements. In January 2017, the FASB issued the update “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” The update eliminates Step 2 from the goodwill impairment test. The annual, or interim, goodwill impairment test is performed by comparing the fair value of a reporting unit with its carrying amount. An impairment charge should be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. In addition, income tax effects from any tax deductible goodwill on the carrying amount of the reporting unit should be considered when measuring the goodwill impairment loss, if applicable. The update also eliminates the requirements for any reporting unit with a zero or negative carrying amount to perform a qualitative assessment and, if it fails that qualitative test, to perform Step 2 of the goodwill impairment test. An entity still has the option to perform the qualitative assessment for a reporting unit to determine if the quantitative impairment test is necessary. The amendments should be applied on a prospective basis. The update is effective for fiscal and interim periods beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. Management has determined that this update would have had no impact on the consolidated financial statements for the periods presented if it had been effective during those periods.
Subsequent Events	Subsequent Events.

Summary of Significant Accoun26

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Schedule of Other Property and Investments	Other Property and Investments. 2016 2015 Centuri property, equipment, and intangibles $ 451,114 $ 423,369 Centuri accumulated provision for depreciation and amortization (228,374 ) (221,028 ) Net cash surrender value of COLI policies 106,744 99,276 Other property 12,859 11,914 Total $ 342,343 $ 313,531
Schedule of Goodwill	Natural Construction Consolidated (In thousands of dollars) December 31, 2015 $ 10,095 $ 116,050 $ 126,145 Additional goodwill from ETTI acquisition — 10,726 10,726 Foreign currency translation adjustment — 3,112 3,112 December 31, 2016 $ 10,095 $ 129,888 $ 139,983
Estimated Fair Values of Assets Acquired as of Acquisition Date	The final estimated fair values of assets acquired as of May 6, 2016, the acquisition date, are as follows (in millions of dollars): Acquisition Property, plant and equipment $ 4.3 Intangible assets 2.9 Goodwill 10.7 Total assets acquired $ 17.9
Summary of Intangible Assets	Centuri has $37.7 million and $36.8 million of intangible assets (varies due to foreign currency translation) at December 31, 2016 and 2015, respectively, as detailed in the following table (thousands of dollars): December 31, 2016 Gross Carrying Accumulated Net Carrying Customer relationships $ 34,033 $ (3,906 ) $ 30,127 Trade names and trademarks 9,349 (2,565 ) 6,784 Customer contracts backlog 1,656 (1,656 ) — Noncompete agreement 1,029 (271 ) 758 Total $ 46,067 $ (8,398 ) $ 37,669 December 31, 2015 Customer relationships $ 31,226 $ (2,070 ) $ 29,156 Trade names and trademarks 8,621 (1,331 ) 7,290 Customer contracts backlog 1,606 (1,606 ) — Noncompete agreement 437 (110 ) 327 Total $ 41,890 $ (5,117 ) $ 36,773
Schedule of Estimated Future Amortization of Intangible Assets	The intangible assets (other than goodwill and software-related intangibles) are included in Other property and investments in the Consolidated Balance Sheets. The estimated future amortization of the intangible assets for the next five years is as follows (in thousands): 2017 $ 3,339 2018 3,126 2019 2,463 2020 2,395 2021 2,269
Schedule of Capitalized and Debt Portion of AFUDC	Utility plant construction costs, including AFUDC, are recovered in authorized rates through depreciation when completed projects are placed into operation, and general rate relief is requested and granted. 2016 2015 2014 (In thousands) AFUDC: Debt portion $ 1,175 $ 1,666 $ 1,228 Equity portion 2,289 3,008 1,995 AFUDC capitalized as part of utility plant $ 3,464 $ 4,674 $ 3,223 AFUDC rate 7.35 % 7.32 % 7.73 %
Other Income (Deductions)	The following table provides the composition of significant items included in Other income (deductions) on the consolidated statements of income (thousands of dollars): 2016 2015 2014 Change in COLI policies $ 7,400 $ (500 ) $ 5,300 Interest income 1,849 2,173 2,602 Equity AFUDC 2,289 3,008 1,995 Foreign currency transaction gain (loss) (22 ) (824 ) (178 ) Equity in earnings of unconsolidated investment - Western 69 310 107 Miscellaneous income and (expense) (2,116 ) (1,288 ) (2,719 ) Total other income (deductions) $ 9,469 $ 2,879 $ 7,107
Schedule of Earnings Per Share, Basic and Diluted	A reconciliation of the denominator used in the Basic and Diluted EPS calculations is shown in the following table. 2016 2015 2014 (In thousands) Average basic shares 47,469 46,992 46,494 Effect of dilutive securities: Stock options 1 8 17 Performance shares 124 171 215 Restricted stock units 220 212 218 Average diluted shares 47,814 47,383 46,944

Utility Plant and Leases (Table

Utility Plant and Leases (Tables)	12 Months Ended
	Dec. 31, 2016
Leases [Abstract]
Schedule of Net Utility Plant	Net utility plant as of December 31, 2016 and 2015 was as follows (thousands of dollars): December 31, 2016 2015 Gas plant: Storage $ 24,614 $ 22,944 Transmission 349,981 312,996 Distribution 5,198,531 4,935,730 General 382,084 365,865 Software and software-related intangibles 224,260 203,323 Other 14,094 14,059 6,193,564 5,854,917 Less: accumulated depreciation (2,172,966 ) (2,084,007 ) Acquisition adjustments, net 196 370 Construction work in progress 111,177 119,805 Net utility plant $ 4,131,971 $ 3,891,085
Schedule of Depreciation and Amortization Expense	Depreciation and amortization expense on gas plant, including intangibles, was as follows (thousands of dollars): 2016 2015 2014 Depreciation and amortization expense $ 214,037 $ 201,233 $ 194,360
Schedule of Rental Payments for Operating Leases	The table below presents Southwest’s rental payments and Centuri’s lease payments that are included in operating expenses (in thousands): 2016 2015 2014 Southwest Gas $ 4,357 $ 4,186 $ 5,330 Centuri 53,956 45,849 30,012 Consolidated rental payments/lease expense $ 58,313 $ 50,035 $ 35,342
Schedule of Future Minimum Lease Payments for Operating Leases	The following is a schedule of future minimum lease payments for significant non-cancelable Year Ending December 31, 2017 $ 6,929 2018 4,837 2019 3,449 2020 2,411 2021 1,098 Thereafter 2,730 Total minimum lease payments $ 21,454
Schedule of Capital Leases of Equipment	The amounts associated with capital leases of equipment as of December 31, 2016 and 2015 are as follows (thousands of dollars): December 31, 2016 2015 Capital leased assets, gross $ 3,189 $ 4,584 Less: accumulated amortization (1,172 ) (1,043 ) Capital leased assets, net $ 2,017 $ 3,541
Schedule of Future Minimum Lease Payments for Capital Leases	The following is a schedule of future minimum lease payments for non-cancelable Year Ending December 31, 2017 $ 931 2018 546 2019 84 2020 — 2021 — Thereafter — 1,561 Less: amount representing interest (101 ) Total minimum lease payments $ 1,460

Receivables and Related Allow28

Receivables and Related Allowances (Tables)	12 Months Ended
	Dec. 31, 2016
Receivables [Abstract]
Schedule of Accounts Receivable	The table below contains information about the gas utility customer accounts receivable balance (net of allowance) at December 31, 2016 and 2015, and the percentage of customers in each of the three states. December 31, December 31, Gas utility customer accounts receivable balance (in thousands) $ 111,320 $ 151,775
Schedule of Percent of Customers by State	December 31, Percent of customers by state Arizona 53 % Nevada 37 % California 10 %
Schedule of Allowance for Uncollectibles	Activity in the allowance account for uncollectibles is summarized as follows (thousands of dollars): Allowance for Balance, December 31, 2013 $ 1,725 Additions charged to expense 4,146 Accounts written off, less recoveries (3,616 ) Balance, December 31, 2014 2,255 Additions charged to expense 4,113 Accounts written off, less recoveries (4,098 ) Balance, December 31, 2015 2,270 Additions charged to expense 3,264 Accounts written off, less recoveries (3,010 ) Balance, December 31, 2016 $ 2,524

Regulatory Assets and Liabili29

Regulatory Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2016
Regulated Operations [Abstract]
Schedule of Regulatory Assets and Liabilities	The following table represents existing regulatory assets and liabilities (thousands of dollars): December 31, 2016 2015 Regulatory assets: Accrued pension and other postretirement benefit costs (1) $ 379,063 $ 384,647 Unrealized net loss on non-trading — 5,486 Deferred purchased gas costs (3) 2,608 3,591 Accrued purchased gas costs (4) 37,100 — Unamortized premium on reacquired debt (5) 21,975 21,511 Accrued absence time (9) 13,440 13,240 Other (6) 23,557 59,782 477,743 488,257 Regulatory liabilities: Deferred purchased gas costs (3) (90,476 ) (45,601 ) Accumulated removal costs (308,000 ) (303,000 ) Accrued purchased gas costs (4) — (10,400 ) Unrealized net gain on non-trading (4,377 ) — Unamortized gain on reacquired debt (7) (9,789 ) (10,325 ) Other (8) (24,659 ) (36,631 ) Net regulatory assets $ 40,442 $ 82,300 (1) Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovery period is greater than five years. (See Note 10). (2) The following table details the regulatory assets/(liabilities) offsetting the derivatives (Swaps) at fair value in the Consolidated Balance Sheets (thousands of dollars). The actual amounts, when realized at settlement, become a component of purchased gas costs under Southwest’s purchased gas adjustment (“PGA”) mechanisms. (See Note 13). Instrument Balance Sheet Location 2016 2015 Swaps Deferred charges and other assets $ — $ 1,219 Swaps Prepaids and other current assets — 4,267 Swaps Other current liabilities (3,532 ) — Swaps Other deferred credits (845 ) — (3) Balance recovered or refunded on an ongoing basis with interest. (4) Asset included in Prepaids and other current assets and liability included in Other current liabilities on the Consolidated Balance Sheets. Balance recovered or refunded on an ongoing basis. (5) Included in Deferred charges and other assets on the Consolidated Balance Sheets. Recovered over life of debt instruments. (6) The following table details the components of Other regulatory assets which are included in either Prepaids and other current assets or Deferred charges and other assets on the Consolidated Balance Sheets (as indicated). Recovery periods vary. Margin tracking/decoupling mechanisms are alternative revenue programs and revenue associated with under-collections (for the difference between authorized margin levels and amounts billed to customers through rates currently) are recognized as revenue so long as recovery is expected to take place within 24 months.
Schedule of Components of Other regulatory assets which are included in either Prepaids and other current assets or Deferred charges and other assets on the Consolidated Balance Sheets	Other Regulatory Assets 2016 2015 State mandated public purpose programs (including low income and conservation programs) (a) (f) $ 7,096 $ 18,101 Margin and interest-tracking accounts (b) (f) 3,517 30,339 Infrastructure replacement programs and similar (c) (f) 6,976 6,947 Environmental compliance programs (d) (f) 4,329 2,300 Other (e) 1,639 2,095 $ 23,557 $ 59,782 a) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets; 2015 included in Deferred charges and other assets on the Consolidated Balance Sheets. b) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets; 2015 included in Prepaids and other current assets on the Consolidated Balance Sheets ($11 million) and Deferred charges and other assets on the Consolidated Balance Sheets ($19.3 million). c) Included in Deferred charges and other assets on the Consolidated Balance Sheets with the exception of $6,000 in 2016 that is included in Prepaids and other current assets on the Consolidated Balance Sheets. d) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets ($3.8 million) and Deferred charges and other assets on the Consolidated Balance Sheets ($500,000); 2015 included in Prepaids and other current assets on the Consolidated Balance Sheets ($1.8 million) and Deferred charges and other assets on the Consolidated Balance Sheets ($484,000). e) 2016 included in Prepaids and other current assets on the Consolidated Balance Sheets ($622,000) and Deferred charges and other assets on the Consolidated Balance Sheets ($1 million); 2015 included in Deferred charges and other assets on the Consolidated Balance Sheets. f) Balance recovered or refunded on an ongoing basis, generally with interest.
Schedule of Components of Other Regulatory Liabilities which are Included in either Other Current Liabilities or Deferred Credits and Other Liabilities on Consolidated Balance Sheets	Other Regulatory Liabilities 2016 2015 State mandated public purpose programs (including low income and conservation programs) (a) (d) $ (7,101 ) $ (4,888 ) Margin and interest-tracking accounts (a) (d) (3,668 ) (20,191 ) Environmental compliance programs (b) (d) (4,469 ) (2,252 ) Regulatory offsets to deferred tax balances (c) (3,390 ) (4,866 ) Regulatory accounts for differences related to pension funding (c) (2,284 ) (1,363 ) Income tax and gross-up (c) (3,203 ) (3,067 ) Other (d) (e) (544 ) (4 ) $ (24,659 ) $ (36,631 ) a) 2016 included in Other current liabilities on the Consolidated Balance Sheets; 2015 included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. b) Included in Other current liabilities on the Consolidated Balance Sheets. c) Included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets. d) Balance recovered or refunded on an ongoing basis, generally with interest. e) 2016 included in Other current liabilities on the Consolidated Balance Sheets ($536,000) and in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets ($8,000); 2015 included in Other deferred credits and other long-term liabilities on the Consolidated Balance Sheets.

Other Comprehensive Income an30

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") (Tables)	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss)	Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Thousands of dollars) 2016 2015 2014 Before- Tax Amount Tax (Expense) or Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Before- Tax Amount Tax (Expense) or Benefit (1) Net-of- Tax Amount Defined benefit pension plans: Net actuarial gain/(loss) $ (22,770 ) $ 8,652 $ (14,118 ) $ (30,519 ) $ 11,597 $ (18,922 ) $ (173,646 ) $ 65,985 $ (107,661 ) Amortization of prior service cost 1,335 (507 ) 828 1,335 (507 ) 828 355 (135 ) 220 Amortization of net actuarial (gain)/loss 27,066 (10,285 ) 16,781 34,381 (13,065 ) 21,316 23,656 (8,989 ) 14,667 Prior service cost — — — — — — (6,661 ) 2,531 (4,130 ) Regulatory adjustment (5,584 ) 2,122 (3,462 ) (5,646 ) 2,146 (3,500 ) 140,308 (53,317 ) 86,991 Pension plans other comprehensive income (loss) 47 (18 ) 29 (449 ) 171 (278 ) (15,988 ) 6,075 (9,913 ) Forward-starting interest rate swaps (“FSIRS”) (designated hedging activities): Amounts reclassified into net income 3,345 (1,270 ) 2,075 3,344 (1,271 ) 2,073 3,345 (1,272 ) 2,073 FSIRS other comprehensive income (loss) 3,345 (1,270 ) 2,075 3,344 (1,271 ) 2,073 3,345 (1,272 ) 2,073 Foreign currency translation adjustments: Translation adjustments 161 — 161 (1,954 ) — (1,954 ) (659 ) — (659 ) Foreign currency other comprehensive income (loss) 161 — 161 (1,954 ) — (1,954 ) (659 ) — (659 ) Total other comprehensive income (loss) $ 3,553 $ (1,288 ) $ 2,265 $ 941 $ (1,100 ) $ (159 ) $ (13,302 ) $ 4,803 $ (8,499 ) (1) Tax amounts are calculated using a 38% rate. Management has elected to indefinitely reinvest the earnings of Centuri’s Canadian subsidiaries in Canada, thus preventing deferred taxes on such earnings. As a result of this assertion, management is not recognizing any tax effect or presenting a tax expense or benefit for the currency translation adjustment amount reported in Other Comprehensive Income, as repatriation of earnings is not anticipated.
Schedule of Estimated Amounts Amortized from Accumulated Other Comprehensive Income or Regulatory Assets into Net Periodic Benefit Cost	The estimated amounts that will be amortized from accumulated other comprehensive income or regulatory assets into net periodic benefit cost over the next year are summarized below (in thousands): Retirement plan net actuarial loss $ 24,000 SERP net actuarial loss 1,500 PBOP prior service cost 1,300
Rollforward of Accumulated Other Comprehensive Income	The following table represents a rollforward of AOCI, presented on the Company’s Consolidated Balance Sheets and its Consolidated Statements of Equity: AOCI—Rollforward (Thousands of dollars) Defined Benefit Plans (Note 10) FSIRS (Note 13) Foreign Currency Items Before- Tax Tax After- Tax Before- Tax Tax After- Tax Before- Tax Tax After- Tax AOCI Beginning Balance AOCI December 31, 2015 $ (57,660 ) $ 21,911 $ (35,749 ) $ (19,344 ) $ 7,350 $ (11,994 ) $ (2,525 ) $ — $ (2,525 ) $ (50,268 ) Net actuarial gain/(loss) (22,770 ) 8,652 (14,118 ) — — — — — — (14,118 ) Translation adjustments — — — — — — 161 — 161 161 Other comprehensive income before reclassifications (22,770 ) 8,652 (14,118 ) — — — 161 — 161 (13,957 ) FSIRS amounts reclassified from AOCI (1) — — — 3,345 (1,270 ) 2,075 — — — 2,075 Amortization of prior service cost (2) 1,335 (507 ) 828 — — — — — — 828 Amortization of net actuarial loss (2) 27,066 (10,285 ) 16,781 — — — — — — 16,781 Regulatory adjustment (3) (5,584 ) 2,122 (3,462 ) — — — — — — (3,462 ) Net current period other comprehensive income (loss) 47 (18 ) 29 3,345 (1,270 ) 2,075 161 — 161 2,265 Less: Translation adjustment attributable to redeemable noncontrolling interest — — — — — — 5 — 5 5 Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation 47 (18 ) 29 3,345 (1,270 ) 2,075 156 — 156 2,260 Ending Balance AOCI December 31, 2016 $ (57,613 ) $ 21,893 $ (35,720 ) $ (15,999 ) $ 6,080 $ (9,919 ) $ (2,369 ) $ — $ (2,369 ) $ (48,008 ) (1) The FSIRS reclassification amounts are included in the Net interest deductions line item on the Consolidated Statements of Income. (2) These AOCI components are included in the computation of net periodic benefit cost (see Note 10 – Pension and Other Postretirement Benefits (3) The regulatory adjustment represents the portion of the activity above that is expected to be recovered through rates in the future (the related regulatory asset is included in the Deferred charges and other assets line item on the Consolidated Balance Sheets). (4) Tax amounts are calculated using a 38% rate.
Amount Recognized Before Income Tax in Accumulated Other Comprehensive Income	The following table represents amounts (before income tax impacts) included in Accumulated other comprehensive income (in the table above), that have not yet been recognized in net periodic benefit cost as of December 31, 2016 and 2015: Amounts Recognized in AOCI (Before Tax) (Thousands of dollars) 2016 2015 Net actuarial (loss) gain $ (430,973 ) $ (435,269 ) Prior service cost (5,703 ) (7,038 ) Less: amount recognized in regulatory assets 379,063 384,647 Recognized in AOCI $ (57,613 ) $ (57,660 )

Long-Term Debt (Tables)

Long-Term Debt (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt	Since Centuri’s debt is not publicly traded, fair values for the secured revolving credit and term loan facility and other debt obligations were based on a conventional discounted cash flow methodology and utilized current market pricing yield curves, across Centuri’s debt maturity spectrum, of other industrial bonds with an assumed credit rating comparable to the Company’s. December 31, 2016 2015 Carrying Amount Market Value Carrying Amount Market Value (Thousands of dollars) Debentures: Notes, 4.45%, due 2020 $ 125,000 $ 129,703 $ 125,000 $ 130,273 Notes, 6.1%, due 2041 125,000 149,734 125,000 141,581 Notes, 3.875%, due 2022 250,000 254,900 250,000 253,600 Notes, 4.875%, due 2043 250,000 266,793 250,000 251,483 Notes, 3.8%, due 2046 300,000 283,029 — — 8% Series, due 2026 75,000 94,691 75,000 97,035 Medium-term notes, 7.59% series, due 2017 25,000 25,040 25,000 26,253 Medium-term notes, 7.78% series, due 2022 25,000 29,290 25,000 29,855 Medium-term notes, 7.92% series, due 2027 25,000 31,905 25,000 31,890 Medium-term notes, 6.76% series, due 2027 7,500 8,769 7,500 8,684 Unamortized discount and debt issuance costs (9,931 ) (6,137 ) 1,197,569 901,363 Revolving credit facility and commercial paper 5,000 5,000 150,000 150,000 Industrial development revenue bonds: Variable-rate bonds: Tax-exempt 50,000 50,000 50,000 50,000 2003 Series A, due 2038 50,000 50,000 50,000 50,000 2008 Series A, due 2038 50,000 50,000 50,000 50,000 2009 Series A, due 2039 50,000 50,000 50,000 50,000 Fixed-rate bonds: 4.85% 2005 Series A, due 2035 — — 100,000 100,452 4.75% 2006 Series A, due 2036 — — 24,855 25,130 Unamortized discount and debt issuance costs (2,489 ) (3,946 ) 197,511 320,909 Centuri term loan facility 106,700 106,819 112,571 112,665 Unamortized debt issuance costs (516 ) (692 ) 106,184 111,879 Centuri secured revolving credit facility 41,185 41,292 60,627 60,724 Centuri other debt obligations 52,635 52,840 25,901 26,059 1,600,084 1,570,679 Less: current maturities (50,101 ) (19,475 ) Long-term debt, less current maturities $ 1,549,983 $ 1,551,204
Summary of Effective Interest Rates on Variable-Rate IDRBs	The effective interest rates on Southwest’s variable-rate IDRBs are included in the table below: December 31, December 31, 2003 Series A 1.47 % 0.87 % 2008 Series A 1.53 % 0.87 % 2009 Series A 1.43 % 0.75 % Tax-exempt 1.51 % 0.81 %
Estimated Maturities of Long-Term Debt	Estimated maturities of long-term debt for the next five years are (in thousands): 2017 $ 50,101 2018 24,082 2019 134,534 2020 134,452 2021 7,815

Pension and Other Postretirem32

Pension and Other Postretirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2016
Compensation and Retirement Disclosure [Abstract]
Cost of Retirement Plan	The cost of the plan is disclosed below (in thousands): 2016 2015 2014 Employee Investment Plan cost $ 4,976 $ 5,072 $ 4,816
Schedule of Percentage Range of Target Portfolio	At December 31, 2016, the percentage ranges of the target portfolio are: Type of Investment Percentage Range Equity securities 63 to 67 Debt securities 33 to 37 Other up to 1
Schedule of Assumptions Used	The rates are presented in the table below: December 31, 2016 December 31, 2015 Discount rate 4.50 % 4.50 % Weighted-average rate of compensation increase 3.25 % 3.25 % Asset return assumption 7.00 % 7.25 %
Schedule of Amounts Recognized in Balance Sheet and Income Statement	The following table sets forth the retirement plan, SERP, and PBOP funded statuses and amounts recognized on the Consolidated Balance Sheets and Consolidated Statements of Income. 2016 2015 Qualified Retirement Plan SERP PBOP Qualified Retirement Plan SERP PBOP (Thousands of dollars) Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO) $ 1,044,817 $ 42,720 $ 72,632 $ 1,060,240 $ 41,176 $ 72,202 Service cost 22,833 331 1,499 25,123 320 1,641 Interest cost 46,027 1,859 3,180 44,229 1,695 2,999 Actuarial loss (gain) 8,550 1,347 (2,060 ) (44,553 ) 2,322 (3,251 ) Benefits paid (73,874 ) (2,946 ) (1,386 ) (40,222 ) (2,793 ) (959 ) Benefit obligation at end of year (PBO/PBO/APBO) 1,048,353 43,311 73,865 1,044,817 42,720 72,632 Change in plan assets Market value of plan assets at beginning of year 736,880 — 43,584 754,796 — 44,892 Actual return on plan assets 39,956 — 4,818 (13,694 ) — (1,034 ) Employer contributions 36,000 2,946 — 36,000 2,793 — Benefits paid (73,874 ) (2,946 ) (289 ) (40,222 ) (2,793 ) (274 ) Market value of plan assets at end of year 738,962 — 48,113 736,880 — 43,584 Funded status at year end $ (309,391 ) $ (43,311 ) $ (25,752 ) $ (307,937 ) $ (42,720 ) $ (29,048 ) Weighted-average assumptions (benefit obligation) Discount rate 4.50 % 4.50 % 4.50 % 4.50 % 4.50 % 4.50 % Weighted-average rate of compensation increase 3.25 % 3.25 % N/A 3.25 % 3.25 % N/A
Schedule of Accumulated Benefit Obligation	The accumulated benefit obligation for the retirement plan and the SERP is presented below (in thousands): December 31, 2016 December 31, 2015 Retirement plan $ 939,002 $ 922,992 SERP 40,852 39,270
Schedule of Expected Benefit Payments	Benefits expected to be paid for pension, SERP, and PBOP over the next 10 years are as follows (in millions): 2017 2018 2019 2020 2021 2022-2026 Pension $ 48.6 $ 50.1 $ 51.5 $ 53.2 $ 55.1 $ 294.2 SERP 2.9 2.9 2.9 2.9 2.9 14.4 PBOP 4.1 4.3 4.4 4.5 4.5 20.6
Schedule of Net Periodic Benefit Cost and Weighted-Average Assumptions	Components of net periodic benefit cost Qualified Retirement Plan SERP PBOP 2016 2015 2014 2016 2015 2014 2016 2015 2014 (Thousands of dollars) Service cost $ 22,833 $ 25,123 $ 21,360 $ 331 $ 320 $ 292 $ 1,499 $ 1,641 $ 1,101 Interest cost 46,027 44,229 43,440 1,859 1,695 1,745 3,180 2,999 2,829 Expected return on plan assets (56,558 ) (57,808 ) (53,342 ) — — — (3,149 ) (3,464 ) (3,264 ) Amortization of prior service cost — — — — — — 1,335 1,335 355 Amortization of net actuarial loss 25,266 32,743 22,873 1,383 1,293 783 417 345 — Net periodic benefit cost $ 37,568 $ 44,287 $ 34,331 $ 3,573 $ 3,308 $ 2,820 $ 3,282 $ 2,856 $ 1,021 Weighted-average assumptions (net benefit cost) Discount rate 4.50 % 4.25 % 5.00 % 4.50 % 4.25 % 5.00 % 4.50 % 4.25 % 5.00 % Expected return on plan assets 7.25 % 7.75 % 7.75 % N/A N/A N/A 7.25 % 7.75 % 7.75 % Weighted-average rate of compensation increase 3.25 % 2.75 % 3.25 % 3.25 % 2.75 % 3.25 % N/A N/A N/A
Schedule of Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income	Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income 2016 2015 2014 Total Qualified SERP PBOP Total Qualified SERP PBOP Total Qualified SERP PBOP (Thousands of dollars) Net actuarial loss (gain) (a) $ 22,770 $ 25,153 $ 1,347 $ (3,730 ) $ 30,519 $ 26,949 $ 2,322 $ 1,248 $ 173,646 $ 163,215 $ 5,460 $ 4,971 Amortization of prior service cost (b) (1,335 ) — — (1,335 ) (1,335 ) — — (1,335 ) (355 ) — — (355 ) Amortization of net actuarial loss (b) (27,066 ) (25,266 ) (1,383 ) (417 ) (34,381 ) (32,743 ) (1,293 ) (345 ) (23,656 ) (22,872 ) (784 ) — Prior service cost — — — — — — — — 6,661 — — 6,661 Regulatory adjustment 5,584 102 — 5,482 5,646 5,214 — 432 (140,308 ) (129,031 ) — (11,277 ) Recognized in other comprehensive (income) loss (47 ) (11 ) (36 ) — 449 (580 ) 1,029 — 15,988 11,312 4,676 — Net periodic benefit costs recognized in net income 44,423 37,568 3,573 3,282 50,451 44,287 3,308 2,856 38,172 34,331 2,820 1,021 Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss $ 44,376 $ 37,557 $ 3,537 $ 3,282 $ 50,900 $ 43,707 $ 4,337 $ 2,856 $ 54,160 $ 45,643 $ 7,496 $ 1,021
Schedule of Fair Value of Plan Assets	The following table sets forth, by level within the three-level fair value hierarchy, the fair values of the assets of the qualified pension plan and the PBOP as of December 31, 2016 and December 31, 2015. The change in the types of pension investment holdings between years is due to the engagement of Russell and the subsequent transition of pension investments to Russell funds. The transition efforts consolidated the majority of the pension plan investments into private commingled equity and fixed income funds. The SERP has no assets. December 31, 2016 December 31, 2015 Qualified PBOP Total Qualified PBOP Total Assets at fair value (thousands of dollars): Level 1 – Quoted prices in active markets for identical financial assets Common stock Agriculture $ — $ — $ — $ 7,021 $ 209 $ 7,230 Capital equipment — — — 533 16 549 Chemicals/materials — — — 3,304 98 3,402 Consumer goods — — — 41,035 1,221 42,256 Energy and mining — — — 11,066 329 11,395 Finance/insurance — — — 29,957 892 30,849 Healthcare — — — 37,930 1,129 39,059 Information technology — — — 29,229 870 30,099 Services — — — 12,341 367 12,708 Telecommunications/internet/media — — — 25,883 770 26,653 Other — — — 9,043 269 9,312 Real estate investment trusts — — — 5,010 149 5,159 Mutual funds — 24,922 24,922 87,483 23,985 111,468 Government fixed income securities — — — 33,482 996 34,478 Futures contracts — — — (7 ) — (7 ) Total Level 1 Assets (1) $ — $ 24,922 $ 24,922 $ 333,310 $ 31,300 $ 364,610 Level 2 – Significant other observable inputs Private commingled equity funds (2) International $ 290,668 $ 9,140 $ 299,808 $ — $ — $ — Large and medium capitalization 121,434 3,819 125,253 — — — Small capitalization 25,947 816 26,763 — — — Emerging markets 45,309 1,424 46,733 — — — Private commingled fixed income funds (3) U.S. corporate bonds 161,086 5,066 166,152 — — — U.S. debt market long duration 77,349 2,432 79,781 — — — U.S. Treasury securities 8,665 272 8,937 — — — Pooled funds and mutual funds 4,889 216 5,105 14,808 796 15,604 Government fixed income and mortgage backed securities 167 5 172 49,571 1,475 51,046 Corporate fixed income securities Asset-backed and mortgage-backed — — — 23,542 701 24,243 Banking — — — 20,857 621 21,478 Insurance — — — 4,896 146 5,042 Utilities — — — 3,826 114 3,940 Other — — — 30,995 922 31,917 Real estate investment trusts — — — 1,949 58 2,007 State and local obligations — — — 950 28 978 Preferred securities — — — 554 17 571 Convertible securities — — — 196 6 202 Total Level 2 assets (4) $ 735,514 $ 23,190 $ 758,704 $ 152,144 $ 4,884 $ 157,028 Total Plan assets at fair value $ 735,514 $ 48,112 $ 783,626 $ 485,454 $ 36,184 $ 521,638 Commingled equity funds (5) — — — 250,511 7,455 257,966 Insurance company general account contracts (6) 3,448 — 3,448 3,719 — 3,719 Total Plan assets (7) $ 738,962 $ 48,112 $ 787,074 $ 739,684 $ 43,639 $ 783,323 (1) The Mutual funds category above is an intermediate-term bond fund whose manager employs multiple concurrent strategies and takes only moderate risk in each, thereby reducing the risk of poor performance arising from any single source, and a balanced fund that invests in a diversified portfolio of common stocks, preferred stocks and fixed-income securities. Strategies utilized by the bond fund include duration management, yield curve or maturity structuring, sector rotation, and all bottom-up in-house In the prior year, Level 1 also included Common stock, Real Estate Investment Trusts, Mutual funds, and U.S. Government securities listed or regularly traded on a national securities exchange and were valued at quoted market prices as of the last business day of the calendar year. (2) The private commingled equity funds include common collective trusts that invest in a diversified portfolio of domestic and international securities regularly traded on securities exchanges. These funds are shown in the above table at net asset value (“NAV”), which is the value of securities in the fund less the amount of any liabilities outstanding. Investment strategies employed by the funds include: • Domestic equities • International developed countries equities • Emerging markets equities Shares in the private equity commingled funds may be redeemed given one business day notice. While they are private equity funds and reported at NAV, due to the short redemption notice period, the lack of significant redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (level 2) and are also therefore, not excluded from the body of the fair value table as a reconciling item. Two funds are classified as international funds. One invests in international financial markets, primarily those of developed economies in Europe and the Pacific Basin. The fund invests primarily in equity securities issued by foreign corporations, but may invest in other securities perceived as offering attractive investment return opportunities. The other provides diversified exposure to global equity markets. The fund seeks to provide long-term capital growth by investing primarily in securities listed on the major developed equity markets of the United States, Europe, and Asia, as well as within those listed on emerging country equity markets on a tactical basis. The large and medium capitalization fund is designed to track the performance of the large and medium capitalization companies contained in the index, which represents approximately 90% of the market capitalization of the United States stock market. The small capitalization fund is designed to provide maximum long-term appreciation through investments that are well diversified by industry. The emerging markets fund was developed to invest in emerging market equities worldwide. The purposes of the fund’s operations, “emerging market countries” include every country in the world except the developed markets of the United States, Canada, Japan, Australia, New Zealand, Hong Kong and Singapore, and most countries located in Western Europe. Fund investments are made directly in each country or, where direct investment is inefficient or prohibited, through appropriate financial instruments or participation in commingled funds. (3) The private commingled fixed income funds include domestic fixed income securities. These funds are shown in the above table at NAV. Shares in the private commingled fixed equity funds may be redeemed given one business day notice. While they are private equity funds and reported at NAV, due to the short redemption notice period, the lack of significant redemption fees, the fact that the underlying investments are exchange-traded, and that substantial liabilities do not exist subject to the NAV calculation, these investments are viewed as indirectly observable (level 2) and are also therefore, not excluded from the body of the fair value table as a reconciling item. The U.S. corporate bond fund seeks to provide high quality, mostly corporate bond-based exposure to fixed income securities which closely match those found in discount curves used to value United States pension liabilities. The United States debt market long duration fund provides participation in the full spectrum of investment opportunities in primarily United States debt markets with longer maturities. The fund seeks to offer effective diversification against equities, take advantage of market trading opportunities, and provide a competitive rate of return on assets. The fund’s current duration is close to 14 years. The United States Treasuries securities funds seeks to replicate the risk and return characteristics of the Barclays Treasury U.S. Separate Trading of Registered Interest and Principal of Securities (“STRIPS”) 28-29 (4) With the exception of items (2) and (3), which are discussed in detail above, the current year Level 2 assets consist mainly of pooled funds and mutual funds. These funds are collective short-term funds that invest in Treasury bills and money market funds and are used as a temporary cash repository. In the prior year, the fair value of the Level 2 investments in debt securities with remaining maturities of one year or more was determined by dealers who make markets in such securities or by an independent pricing service, which considers yield or price of bonds of comparable quality, coupon, maturity, and type. (5) In the prior year, the commingled equity funds included private equity funds that invest in domestic and international securities regularly traded on securities exchanges. These funds are shown in the above table at net asset value, which is the value of securities in the fund less the amount of any liabilities outstanding. Investment strategies employed by the funds included: • Domestic large capitalization value equities • International developed countries value and growth equities • Emerging markets equities • International small capitalization equities The terms and conditions under which shares in the commingled equity funds were redeemed varied among the funds; the notice required ranged from one day to 30 days prior to the valuation date (month end). One of the commingled equity funds required the payment of a minimal impact fee to be applied to redemptions and subscriptions of $5 million or greater; the relative fee diminished the greater the transaction. Other such funds imposed fees to recover direct costs incurred by the fund at redemption, but were indeterminable prior to redemption. (6) The insurance company general account contracts are annuity insurance contracts used to pay the pensions of employees who retired prior to 1989. The balance of the account disclosed in the above table is the contract value, which is the result of deposits, withdrawals, and interest credits. (7) In the prior year, the assets in the above table exceeded the market value of plan assets shown in the funded status table by $2,859,000 (qualified retirement plan – $2,803,000, PBOP – $56,000), which includes a payable for securities purchased, partially offset by receivables for interest, dividends, and securities sold.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Stock-Based Plan Compensation Expense, Including Cash Award	The table below shows total stock-based plan compensation expense, including the cash award, which was recognized in the Consolidated Statements of Income (in thousands): 2016 2015 2014 Stock-based compensation plan expense, net of related tax benefits $ 7,185 $ 7,278 $ 8,130 Stock-based compensation plan related tax benefits 4,404 4,461 4,983
Schedule of Stock Options Activity	The following tables summarize the stock option plan activity and related information (thousands of options): 2016 2015 2014 Number Weighted- Number of Weighted- Number of Weighted- Outstanding at the beginning of the year 17 $ 31.64 36 $ 28.97 52 $ 27.57 Exercised during the year (17 ) 31.64 (19 ) 26.69 (16 ) 24.31 Forfeited or expired during the year — — — — — — Outstanding and exercisable at year end — N/A 17 $ 31.64 36 $ 28.97
Schedule of Aggregate Intrinsic Value of Outstanding and Exercisable Options	The aggregate intrinsic value of outstanding and exercisable options, and options that were exercised, are presented in the table below (in thousands): 2016 2015 2014 Outstanding and exercisable $ — $ 394 $ 1,194 Exercised 554 590 451
Summary of Market Prices of Common Stock	December 31, 2016 December 31, 2015 December 31, 2014 Market value of Company stock $ 76.62 $ 55.16 $ 61.81
Schedule of Nonvested Performance and Restricted Stock Unit Plans	The following table summarizes the activity of the performance share stock and restricted stock/unit plans as of December 31, 2016 (thousands of shares): Performance Weighted- Restricted Weighted- Nonvested/unissued at beginning of year 197 $ 50.63 228 $ 44.36 Granted 44 59.05 73 60.39 Dividends 5 6 Forfeited or expired — — — — Vested and issued* (78 ) 41.82 (45 ) 51.98 Nonvested/unissued at December 31, 2016 168 $ 55.62 262 $ 46.41 * Includes shares for retiree payouts and those converted for taxes.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations	The following is a summary of income before taxes and noncontrolling interest for domestic and foreign operations (thousands of dollars): Year ended December 31, 2016 2015 2014 U.S. $ 218,810 $ 221,660 $ 221,471 Foreign 12,713 (2,328 ) (1,950 ) Total income before income taxes $ 231,523 $ 219,332 $ 219,521
Summary of Income Tax Expense (Benefit)	Income tax expense (benefit) consists of the following (thousands of dollars): Year Ended December 31, 2016 2015 2014 Current: Federal $ 541 $ 21,321 $ 1,739 State 5,748 9,899 5,073 Foreign 4,298 650 2,193 10,587 31,870 9,005 Deferred: Federal 68,270 51,132 71,439 State 140 (2,574 ) 614 Foreign (529 ) (526 ) (2,685 ) 67,881 48,032 69,368 Total income tax expense $ 78,468 $ 79,902 $ 78,373
Significant Components of Deferred Income Tax Expense (Benefit)	Deferred income tax expense (benefit) consists of the following significant components (thousands of dollars): Year Ended December 31, 2016 2015 2014 Deferred federal and state: Property-related items $ 76,217 $ 65,931 $ 52,814 Purchased gas cost adjustments 361 (32,993 ) 15,049 Employee benefits (1,327 ) 623 109 All other deferred (6,532 ) 15,332 2,257 Total deferred federal and state 68,719 48,893 70,229 Deferred ITC, net (838 ) (861 ) (861 ) Total deferred income tax expense $ 67,881 $ 48,032 $ 69,368
Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate	A reconciliation of the U.S. federal statutory rate to the consolidated effective tax rate for 2014, 2015, and 2016 (and the sources of these differences and the effect of each) are summarized as follows: Year Ended December 31, 2016 2015 2014 U.S. federal statutory income tax rate 35.0 % 35.0 % 35.0 % Net state taxes 1.4 1.8 1.9 Property-related items — 0.1 0.1 Tax credits (0.4 ) (0.4 ) (0.5 ) Company owned life insurance (1.2 ) 0.1 (1.0 ) All other differences (0.9 ) (0.2 ) 0.2 Consolidated effective income tax rate 33.9 % 36.4 % 35.7 %
Deferred Tax Assets and Liabilities	Deferred tax assets and liabilities consist of the following (thousands of dollars): December 31, 2016 2015 Deferred tax assets: Deferred income taxes for future amortization of ITC $ 1,094 $ 1,614 Employee benefits 38,231 36,923 Alternative minimum tax credit 4,827 4,809 Net operating losses and credits 1,204 868 Interest rate swap 6,080 7,351 Other 18,415 24,636 Valuation allowance (495 ) (499 ) 69,356 75,702 Deferred tax liabilities: Property-related items, including accelerated depreciation 872,136 794,850 Regulatory balancing accounts 1,104 743 Unamortized ITC 1,710 2,549 Debt-related costs 5,712 5,497 Intangibles 8,803 9,547 Other 19,256 31,533 908,721 844,719 Net noncurrent deferred tax liabilities $ 839,365 $ 769,017
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (thousands of dollars): 2016 2015 Unrecognized tax benefits at beginning of year $ 296 $ 305 Gross increases – tax positions in prior period 897 — Gross decreases – tax positions in prior period — (9 ) Gross increases – current period tax positions 38 — Gross decreases – current period tax positions — — Settlements — — Lapse in statute of limitations — — Unrecognized tax benefits at end of year $ 1,231 $ 296

Derivatives and Fair Value Me35

Derivatives and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Notional Amounts under Swaps Contracts	The differential is calculated based on the notional amounts under the contracts, which are detailed in the table below (thousands of dekatherms): December 31, 2016 December 31, 2015 Contract notional amounts 10,543 7,407
Amount of Gain or Losses Recognized in Income on Derivatives	The following table sets forth the gains and (losses) recognized on Southwest’s Swaps (derivatives) for the years ended December 31, 2016, 2015, and 2014 and their location in the Consolidated Statements of Income: Gains (losses) recognized in income for derivatives not designated as hedging instruments: (Thousands of dollars) Instrument Location of Gain or (Loss) 2016 2015 2014 Swaps Net cost of gas sold $ 5,006 $ (7,598 ) $ (2,363 ) Swaps Net cost of gas sold (5,006 )* 7,598 * 2,363 * Total $ — $ — $ — * Represents the impact of regulatory deferral accounting treatment under U.S. GAAP for rate-regulated entities.
Fair Values of Swaps in Consolidated Balance Sheets	The following table sets forth the fair values of the Swaps and their location in the Consolidated Balance Sheets (thousands of dollars): Fair values of derivatives not designated as hedging instruments: December 31, 2016 Instrument Balance Sheet Location Asset Liability Net Swaps Deferred charges and other assets $ 899 $ (54 ) $ 845 Swaps Prepaids and other current assets 3,551 (19 ) 3,532 Total $ 4,450 $ (73 ) $ 4,377 December 31, 2015 Instrument Balance Sheet Location Asset Liability Net Swaps Other current liabilities $ — $ (4,267 ) $ (4,267 ) Swaps Other deferred credits 4 (1,223 ) (1,219 ) Total $ 4 $ (5,490 ) $ (5,486 )
Paid to and Received from Counterparties for Settlements of Matured Swaps	The following table presents the amounts paid to and received from counterparties for settlements of matured Swaps. Year ended Year ended Year ended (Thousands of dollars) Paid to counterparties $ 5,583 $ 7,537 $ 829 Received from counterparties $ 726 $ — $ 4,713
Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets	The following table details the regulatory assets/(liabilities) offsetting the derivatives at fair value in the Consolidated Balance Sheets (thousands of dollars). December 31, 2016 Instrument Balance Sheet Location Net Total Swaps Other deferred credits $ (845 ) Swaps Other current liabilities (3,532 ) December 31, 2015 Instrument Balance Sheet Location Net Total Swaps Prepaids and other current assets $ 4,267 Swaps Deferred charges and other assets 1,219
Significant Other Observable Inputs	The following table sets forth, by level within the three-level fair value hierarchy that ranks the inputs used to measure fair value by their reliability, financial assets and liabilities that were accounted for at fair value (see Note 10 – Pension and Other Post Retirement Benefits for definitions of the levels of the fair value hierarchy): Level 2 – Significant other observable inputs December 31, 2016 December 31, 2015 (Thousands of dollars) Assets at fair value: Prepaids and other current assets – Swaps $ 3,532 $ — Deferred charges and other assets – Swaps 845 — Liabilities at fair value: Other current liabilities – Swaps — (4,267 ) Other deferred credits – Swaps — (1,219 ) Net Assets (Liabilities) $ 4,377 $ (5,486 )

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2016
Segment Reporting [Abstract]
Accounts Receivable for Services	Accounts receivable for these services, which are not eliminated during consolidation, are presented in the table below (in thousands). December 31, 2016 December 31, 2015 Accounts receivable for Centuri services $ 10,585 $ 10,006
Schedule of Revenues by Geographic Area	The following table presents the amount of revenues for both segments by geographic area (thousands of dollars): December 31, December 31, December 31, Revenues (a) United States $ 2,256,600 $ 2,289,133 $ 2,069,513 Canada 203,890 174,492 52,194 Total $ 2,460,490 $ 2,463,625 $ 2,121,707 (a) Revenues are attributed to countries based on the location of customers.
Schedule of Segment Reporting Information	The financial information pertaining to the natural gas operations and construction services segments for each of the three years in the period ended December 31, 2016 is as follows (thousands of dollars): 2016 Gas Construction Adjustments Total Revenues from unaffiliated customers $ 1,321,412 $ 1,040,957 $ 2,362,369 Intersegment sales — 98,121 98,121 Total $ 1,321,412 $ 1,139,078 $ 2,460,490 Interest revenue $ 1,848 $ 1 $ 1,849 Interest expense $ 66,997 $ 6,663 $ 73,660 Depreciation and amortization $ 233,463 $ 55,669 $ 289,132 Income tax expense $ 58,584 $ 19,884 $ 78,468 Segment net income $ 119,423 $ 32,618 $ 152,041 Segment assets $ 5,001,756 $ 579,370 $ 5,581,126 Capital expenditures $ 457,120 $ 72,411 $ 529,531 2015 Gas Construction Adjustments Total Revenues from unaffiliated customers $ 1,454,639 $ 904,870 $ 2,359,509 Intersegment sales — 104,116 104,116 Total $ 1,454,639 $ 1,008,986 $ 2,463,625 Interest revenue $ 1,754 $ 419 $ 2,173 Interest expense $ 64,095 $ 7,784 $ 71,879 Depreciation and amortization $ 213,455 $ 56,656 $ 270,111 Income tax expense $ 61,355 $ 18,547 $ 79,902 Segment net income $ 111,625 $ 26,692 $ 138,317 Segment assets $ 4,822,845 $ 535,840 $ 5,358,685 Capital expenditures $ 438,289 $ 49,711 $ 488,000 2014 Gas Construction Adjustments (a) Total Revenues from unaffiliated customers $ 1,382,087 $ 647,432 $ 2,029,519 Intersegment sales — 92,188 92,188 Total $ 1,382,087 $ 739,620 $ 2,121,707 Interest revenue $ 2,596 $ 6 $ 2,602 Interest expense $ 68,299 $ 3,770 $ 72,069 Depreciation and amortization $ 204,144 $ 48,883 $ 253,027 Income tax expense $ 63,597 $ 14,776 $ 78,373 Segment net income $ 116,872 $ 24,254 $ 141,126 Segment assets $ 4,652,307 $ 566,589 $ (10,599 ) $ 5,208,297 Capital expenditures $ 350,025 $ 46,873 $ 396,898 (a) Construction services segment assets included two liabilities that were netted against gas operations segment assets during consolidation in 2014. They are: Income taxes payable of $3.3 million, netted against income taxes receivable, net and deferred income taxes of $1.4 million, netted against deferred income taxes, net. Construction services segment assets exclude a long-term deferred tax benefit of $1.4 million, which was netted against gas operations segment deferred income taxes and investment tax credits, net during consolidation. Gas operations segment assets include a deferred income tax liability of $4.5 million, which was netted against a construction services segment asset for deferred income taxes, net during consolidation.

Quarterly Financial Data (Table

Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	Quarter Ended March 31 June 30 September 30 December 31 (Thousands of dollars, except per share amounts) 2016 Operating revenues $ 731,248 $ 547,748 $ 539,969 $ 641,525 Operating income 134,096 28,116 15,539 117,963 Net income 75,355 9,099 2,907 65,694 Net income attributable to Southwest Gas Corporation 75,446 8,943 2,472 65,180 Basic earnings per common share* 1.59 0.19 0.05 1.37 Diluted earnings per common share* 1.58 0.19 0.05 1.36 2015 Operating revenues $ 734,220 $ 538,604 $ 505,396 $ 685,405 Operating income 129,556 25,047 16,143 117,586 Net income (loss) 71,879 5,063 (4,210 ) 66,698 Net income (loss) attributable to Southwest Gas Corporation 71,983 4,949 (4,734 ) 66,119 Basic earnings (loss) per common share* 1.54 0.11 (0.10 ) 1.40 Diluted earnings (loss) per common share* 1.53 0.10 (0.10 ) 1.38 2014 Operating revenues $ 608,396 $ 453,153 $ 432,475 $ 627,683 Operating income 127,065 26,755 18,290 112,373 Net income 70,697 9,627 1,927 58,897 Net income attributable to Southwest Gas Corporation 70,783 9,627 1,970 58,746 Basic earnings per common share* 1.52 0.21 0.04 1.26 Diluted earnings per common share* 1.51 0.21 0.04 1.25 * The sum of quarterly earnings (loss) per average common share may not equal the annual earnings (loss) per share due to the ongoing change in the weighted-average number of common shares outstanding.

Construction Services Noncont38

Construction Services Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2016
Noncontrolling Interest [Abstract]
Summary of Redeemable Noncontrolling Interest	The following depicts impacts to the balance of the redeemable noncontrolling interest between the indicated periods. Redeemable (Thousands of dollars): Balance, December 31, 2015 $ 16,108 Net Income (loss) attributable to redeemable noncontrolling interest 1,148 Foreign currency exchange translation adjustment 5 Centuri distribution to redeemable noncontrolling interest (439 ) Adjustment to redemption value 5,768 Balance, December 31, 2016 $ 22,590

Summary of Significant Accoun39

Summary of Significant Accounting Policies - Additional Information (Detail)	May 06, 2016USD ($)	Dec. 31, 2016USD ($)SegmentMarkets	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Significant Accounting Policies [Line Items]
Number of segments | Segment		2
Earnings from equity method investment		$ 69,000	$ 310,000	$ 107,000
Non-cash construction advances - non-cash investing item		6,500,000	3,100,000	8,100,000
Non-cash investing outflow activity				10,800,000
Goodwill impairment charges		0	0
Purchase price paid in cash	$ 17,000,000
Business acquisition deferred payment period	4 years
Reduction in depreciation expense due to extension of estimates useful lives		$ 4,000,000
Non-controlling Interest [Member]
Significant Accounting Policies [Line Items]
Non-cash investing outflow activity				$ 18,900,000
IntelliChoice Energy, LLC [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		65.00%
W.S. Nicholls Western Construction LTD. [Member]
Significant Accounting Policies [Line Items]
Investment amount		$ 10,800,000	10,800,000
Dividends		500,000
Exposure to loss as a result of its investment		35,800,000
Earnings from equity method investment		$ 69,000
Matheson-Nicholls Joint Venture [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		50.00%
CCI-TBN Toronto, Inc [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		25.00%
Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Ownership percentage in subsidiary		96.60%
Intangible assets		$ 37,669,000	36,773,000
Centuri Construction Group Inc [Member] | W.S. Nicholls Western Construction LTD. [Member]
Significant Accounting Policies [Line Items]
Subsidiaries holding interest percentage		50.00%
Pooled Funds and Mutual Funds [Member] | Level 2 - Significant Other Observable Inputs [Member]
Significant Accounting Policies [Line Items]
Money market fund investments		$ 5,300,000	250,000
United States [Member] | Centuri Construction Group Inc [Member]
Significant Accounting Policies [Line Items]
Number of major markets in the United States | Markets		20
Gas Pipe Materials and Supplies [Member]
Significant Accounting Policies [Line Items]
Gas pipe materials and operating supplies		$ 30,000,000	$ 24,000,000
Foreign Jurisdiction [Member]
Significant Accounting Policies [Line Items]
Cumulative earnings		$ 5,000,000

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Schedule of Other Property and Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investment [Line Items]
Net cash surrender value of COLI policies	$ 106,744,000	$ 99,276,000
Other property	12,859,000	11,914,000
Total	342,343	313,531
Centuri Construction Group Inc [Member]
Investment [Line Items]
Centuri property, equipment, and intangibles	451,114,000	423,369,000
Centuri accumulated provision for depreciation and amortization	$ (228,374,000)	$ (221,028,000)

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Schedule of Goodwill (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Goodwill [Line Items]
Goodwill, Beginning balance	$ 126,145
Foreign currency translation adjustment	3,112
Goodwill, Ending balance	139,983
ETTI [Member]
Goodwill [Line Items]
Additional goodwill from ETTI acquisition	10,726
Natural Gas Operations [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	10,095
Foreign currency translation adjustment	0
Goodwill, Ending balance	10,095
Natural Gas Operations [Member] | ETTI [Member]
Goodwill [Line Items]
Additional goodwill from ETTI acquisition	0
Construction Services [Member]
Goodwill [Line Items]
Goodwill, Beginning balance	116,050
Foreign currency translation adjustment	3,112
Goodwill, Ending balance	129,888
Construction Services [Member] | ETTI [Member]
Goodwill [Line Items]
Additional goodwill from ETTI acquisition	$ 10,726

Summary of Significant Accoun42

Summary of Significant Accounting Policies - Estimated Fair Values of Assets Acquired as of Acquisition Date (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	May 06, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill	$ 139,983		$ 126,145
ETTI [Member]
Business Acquisition [Line Items]
Property, plant and equipment		$ 4,300
Intangible assets		2,900
Goodwill		10,700
Total assets acquired		$ 17,900

Summary of Significant Accoun43

Summary of Significant Accounting Policies - Summary of Intangible Assets (Detail) - Centuri Construction Group Inc [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 46,067	$ 41,890
Accumulated Amortization	(8,398)	(5,117)
Net Carrying Amount	37,669	36,773
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34,033	31,226
Accumulated Amortization	(3,906)	(2,070)
Net Carrying Amount	30,127	29,156
Trade Names and Trademarks [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	9,349	8,621
Accumulated Amortization	(2,565)	(1,331)
Net Carrying Amount	6,784	7,290
Customer Contracts Backlog [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,656	1,606
Accumulated Amortization	(1,656)	(1,606)
Net Carrying Amount	0	0
Noncompete Agreement [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,029	437
Accumulated Amortization	(271)	(110)
Net Carrying Amount	$ 758	$ 327

Summary of Significant Accoun44

Summary of Significant Accounting Policies - Schedule of Estimated Future Amortization of Intangible Assets (Detail) $ in Thousands	Dec. 31, 2016USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2,017	$ 3,339
2,018	3,126
2,019	2,463
2,020	2,395
2,021	$ 2,269

Summary of Significant Accoun45

Summary of Significant Accounting Policies - Schedule of Capitalized and Debt Portion of AFUDC (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Public Utilities, Allowance for Funds Used During Construction, Net Increase [Abstract]
Debt portion	$ 1,175	$ 1,666	$ 1,228
Equity portion	2,289	3,008	1,995
AFUDC capitalized as part of utility plant	$ 3,464	$ 4,674	$ 3,223
AFUDC rate	7.35%	7.32%	7.73%

Summary of Significant Accoun46

Summary of Significant Accounting Policies - Other Income (Deductions) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Income and Expenses [Abstract]
Change in COLI policies	$ 7,400	$ (500)	$ 5,300
Interest income	1,849	2,173	2,602
Equity AFUDC	2,289	3,008	1,995
Foreign currency transaction gain (loss)	(22)	(824)	(178)
Equity in earnings of unconsolidated investment - Western	69	310	107
Miscellaneous income and (expense)	(2,116)	(1,288)	(2,719)
Total other income (deductions)	$ 9,469	$ 2,879	$ 7,107

Summary of Significant Accoun47

Summary of Significant Accounting Policies - Schedule of Earnings Per Share, Basic and Diluted (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Average basic shares	47,469	46,992	46,494
Stock options	1	8	17
Performance shares	124	171	215
Restricted stock units	220	212	218
Average diluted shares	47,814	47,383	46,944

Utility Plant and Leases - Sche

Utility Plant and Leases - Schedule of Net Utility Plant (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 6,193,564	$ 5,854,917
Less: accumulated depreciation	(2,172,966)	(2,084,007)
Acquisition adjustments, net	196	370
Construction work in progress	111,177	119,805
Net utility plant	4,131,971	3,891,085
Software and Software-related Intangibles [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	224,260	203,323
Storage [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	24,614	22,944
Transmission [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	349,981	312,996
Distribution [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	5,198,531	4,935,730
General [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	382,084	365,865
Other [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Gas plant	$ 14,094	$ 14,059

Utility Plant and Leases - Addi

Utility Plant and Leases - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Annual depreciation and amortization expense percentage	3.60%	3.60%	3.60%
Gas, Transmission and Distribution Plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Annual depreciation and amortization expense percentage	3.30%	3.30%	3.30%
Gas plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of intangibles	$ 14.8	$ 12.7	$ 11.7

Utility Plant and Leases - Sc50

Utility Plant and Leases - Schedule of Depreciation and Amortization Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 289,132	$ 270,111	$ 253,027
Gas plant [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 214,037	$ 201,233	$ 194,360

Utility Plant and Leases - Sc51

Utility Plant and Leases - Schedule of Rental Payments for Operating Leases (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	$ 58,313	$ 50,035	$ 35,342
Southwest Gas [Member]
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	4,357	4,186	5,330
Centuri Construction Group Inc [Member]
Rental Payments And Lease Payments Included In Operating Expenses [Line Items]
Consolidated rental payments/lease expense	$ 53,956	$ 45,849	$ 30,012

Utility Plant and Leases - Sc52

Utility Plant and Leases - Schedule of Future Minimum Lease Payments for Operating Leases (Detail) $ in Thousands	Dec. 31, 2016USD ($)
Leases [Abstract]
2,017	$ 6,929
2,018	4,837
2,019	3,449
2,020	2,411
2,021	1,098
Thereafter	2,730
Total minimum lease payments	$ 21,454

Utility Plant and Leases - Sc53

Utility Plant and Leases - Schedule of Capital Leases of Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Leases [Abstract]
Capital leased assets, gross	$ 3,189	$ 4,584
Less: accumulated amortization	(1,172)	(1,043)
Capital leased assets, net	$ 2,017	$ 3,541

Utility Plant and Leases - Sc54

Utility Plant and Leases - Schedule of Future Minimum Lease Payments for Capital Leases (Detail) $ in Thousands	Dec. 31, 2016USD ($)
Leases [Abstract]
2,017	$ 931
2,018	546
2,019	84
2,020	0
2,021	0
Thereafter	0
Future minimum lease payments due	1,561
Less: amount representing interest	(101)
Total minimum lease payments	$ 1,460

Receivables and Related Allow55

Receivables and Related Allowances - Schedule of Accounts Receivable (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Gas utility customer accounts receivable balance	$ 111,320	$ 151,775

Receivables and Related Allow56

Receivables and Related Allowances - Schedule of Percent of Customers by State (Detail)	12 Months Ended
	Dec. 31, 2016
Arizona [Member]
Receivables And Related Allowances [Line Items]
Percent of customers by state	53.00%
Nevada [Member]
Receivables And Related Allowances [Line Items]
Percent of customers by state	37.00%
California [Member]
Receivables And Related Allowances [Line Items]
Percent of customers by state	10.00%

Receivables and Related Allow57

Receivables and Related Allowances - Schedule of Allowance for Uncollectibles (Detail) - Allowance for Uncollectibles [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Receivables And Related Allowances [Line Items]
Beginning Balance	$ 2,270	$ 2,255	$ 1,725
Additions charged to expense	3,264	4,113	4,146
Accounts written off, less recoveries	(3,010)	(4,098)	(3,616)
Ending Balance	$ 2,524	$ 2,270	$ 2,255

Receivables and Related Allow58

Receivables and Related Allowances - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Receivables And Related Allowances [Line Items]
Customer accounts receivable	$ 285,145	$ 314,512
Centuri Construction Group Inc [Member]
Receivables And Related Allowances [Line Items]
Customer accounts receivable	$ 173,000

Regulatory Assets and Liabili59

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	$ 477,743	$ 488,257
Net regulatory assets	40,442	82,300
Regulatory liabilities, total	(24,659)	(36,631)
Accrued Pension and Other Postretirement Benefit Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	379,063	384,647
Unrealized Net Loss on Non-trading Derivatives (Swaps) [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	0	5,486
Regulatory liabilities, total	(4,377)	0
Deferred Purchased Gas Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	2,608	3,591
Regulatory liabilities, total	(90,476)	(45,601)
Accrued Purchased Gas Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	37,100	0
Regulatory liabilities, total	0	(10,400)
Unamortized Premium on Reacquired Debt [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	21,975	21,511
Accrued Absence Time [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	13,440	13,240
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets, total	23,557	59,782
Accumulated Removal Costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(308,000)	(303,000)
Unamortized Gain on Reacquired Debt [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	(9,789)	(10,325)
Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities, total	$ (24,659)	$ (36,631)

Regulatory Assets and Liabili60

Regulatory Assets and Liabilities - Schedule of Regulatory Assets and Liabilities (Parenthetical) (Detail) - Swaps [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Charges and Other Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value	$ 0	$ 1,219
Prepaids and Other Current Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value	0	4,267
Other Current Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value	(3,532)	0
Other Deferred Credits [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets and liabilities offsetting derivatives at fair value	$ (845)	$ 0

Regulatory Assets and Liabili61

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Assets are Included in either Prepaids and Other Current Assets or Deferred Credits and Other Assets on the Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Regulatory assets	$ 477,743	$ 488,257
State Mandate Public Purpose Programs (Including Low Income and Conservation Programs) [Member]
Regulatory Assets [Line Items]
Regulatory assets	7,096	18,101
Margin and Interest-Tracking Accounts [Member]
Regulatory Assets [Line Items]
Regulatory assets	3,517	30,339
Infrastructure Replacement Programs and Similar [Member]
Regulatory Assets [Line Items]
Regulatory assets	6,976	6,947
Environmental Compliance Programs [Member]
Regulatory Assets [Line Items]
Regulatory assets	4,329	2,300
Other [Member]
Regulatory Assets [Line Items]
Regulatory assets	1,639	2,095
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 23,557	$ 59,782

Regulatory Assets and Liabili62

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Assets are Included in Deferred Credits and Other Assets on the Consolidated Balance Sheets (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Assets [Line Items]
Prepaid and other current assets	$ 136,833	$ 95,199
Deferred charges and other assets	432,234	469,322
Margin and Interest-Tracking Accounts [Member]
Regulatory Assets [Line Items]
Prepaid and other current assets		11,000
Deferred charges and other assets	19,300
Infrastructure Replacement Programs and Similar [Member]
Regulatory Assets [Line Items]
Prepaid and other current assets	6,000
Environmental Compliance Programs [Member]
Regulatory Assets [Line Items]
Prepaid and other current assets	3,800	1,800
Deferred charges and other assets	500,000	$ 484,000
Other Regulatory Assets [Member]
Regulatory Assets [Line Items]
Prepaid and other current assets	622,000
Deferred charges and other assets	$ 1,000

Regulatory Assets and Liabili63

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Liabilities are Included in either Other Current Liabilities or Deferred Credits and Other Liabilities on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ (24,659)	$ (36,631)
State Mandate Public Purpose Programs (Including Low Income and Conservation Programs) [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	(7,101)	(4,888)
Margin and Interest-Tracking Accounts [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	(3,668)	(20,191)
Environmental Compliance Programs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	(4,469)	(2,252)
Regulatory Offsets To Deferred Tax Balances [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	(3,390)	(4,866)
Regulatory Accounts for Differences Related to Pension Funding [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	(2,284)	(1,363)
Income Tax and Gross-Up [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	(3,203)	(3,067)
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ (544)	$ (4)

Regulatory Assets and Liabili64

Regulatory Assets and Liabilities - Schedule of Components of Other Regulatory Liabilities are Included in either Other Current Liabilities or Deferred Credits and Other Liabilities on Consolidated Balance Sheets (Parenthetical) (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Other current liabilities	$ 168,064,000	$ 150,031,000
Other deferred credits and other long-term liabilities	570,252,000	$ 591,558,000
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Other current liabilities	536,000
Other deferred credits and other long-term liabilities	$ 8,000

Other Comprehensive Income an65

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Net actuarial gain/(loss), Before-Tax Amount	$ (22,770)	$ (30,519)	$ (173,646)
Amortization of prior service cost, Before-Tax Amount	1,335	1,335	355
Amortization of net actuarial (gain)/loss, Before-Tax Amount	27,066	34,381	23,656
Prior service cost, Before-Tax Amount	0	0	(6,661)
Regulatory adjustment, Before-Tax Amount	(5,584)	(5,646)	140,308
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	47	(449)	(15,988)
Amounts reclassified into net income, Before-Tax Amount	3,345	3,344	3,345
FSIRS other comprehensive income (loss), Before-Tax Amount	3,345	3,344	3,345
Translation adjustments, Before-Tax Amount	161	(1,954)	(659)
Foreign currency other comprehensive income (loss), Before-Tax Amount	161	(1,954)	(659)
Total other comprehensive income (loss), Before-Tax Amount	3,553	941	(13,302)
Net actuarial gain/(loss), Tax (Expense) or Benefit	8,652	11,597	65,985
Amortization of prior service cost, Tax (Expense) or Benefit	(507)	(507)	(135)
Amortization of net actuarial (gain)/loss, Tax (Expense) or Benefit	(10,285)	(13,065)	(8,989)
Prior service cost, Tax (Expense) or Benefit	0	0	2,531
Regulatory adjustment, Tax (Expense) or Benefit	2,122	2,146	(53,317)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(18)	171	6,075
Amounts reclassified into net income, Tax (Expense) or Benefit	(1,270)	(1,271)	(1,272)
FSIRS other comprehensive income (loss), Tax (Expense) or Benefit	(1,270)	(1,271)	(1,272)
Translation adjustments, Tax (Expense) or Benefit	0	0	0
Foreign currency other comprehensive income (loss), Tax (Expense) or Benefit	0	0	0
Total other comprehensive income (loss), Tax (Expense) or Benefit	(1,288)	(1,100)	4,803
Net actuarial gain/(loss), Net-of-Tax Amount	(14,118)	(18,922)	(107,661)
Amortization of prior service cost, Net-of-Tax Amount	828	828	220
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	16,781	21,316	14,667
Prior service cost, Net-of-Tax Amount	0	0	(4,130)
Regulatory adjustment, Net-of-Tax Amount	(3,462)	(3,500)	86,991
Pension plans other comprehensive income (loss), Net-of-Tax Amount	29	(278)	(9,913)
Amounts reclassified into net income (Notes 5 and 13)	2,075	2,073	2,073
FSIRS other comprehensive income (loss), Net-of-Tax Amount	2,075	2,073	2,073
Translation adjustments, Net-of-Tax Amount	161	(1,954)	(659)
Foreign currency other comprehensive income (loss), Net-of-Tax Amount	161	(1,954)	(659)
Total other comprehensive income (loss), net of tax	$ 2,265	$ (159)	$ (8,499)

Other Comprehensive Income an66

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Related Tax Effects Allocated to Each Component of Other Comprehensive Income (Loss) (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2016USD ($)
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%
Tax expense or benefit for currency translation adjustment	$ 0

Other Comprehensive Income an67

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Schedule of Estimated Amounts Amortized from Accumulated Other Comprehensive Income or Regulatory Assets into Net Periodic Benefit Cost (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	$ 24,000
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Retirement plan net actuarial loss	1,500
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
PBOP prior service cost	$ 1,300

Other Comprehensive Income an68

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Additional Information (Detail) $ in Millions	Dec. 31, 2016USD ($)
Equity [Abstract]
Amount of FSIRS existing AOCI losses expected to reclassified income in next twelve months	$ 2.1

Other Comprehensive Income an69

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Rollforward of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Beginning Balance AOCI December 31, 2015, Defined Benefit Plans Before-Tax	$ (57,660)
Net actuarial gain/(loss), Defined Benefit Plans Before-Tax	(22,770)	$ (30,519)	$ (173,646)
Translation adjustments, Defined Benefit Plans Before-Tax	0
Other comprehensive income before reclassifications, Defined Benefit Plans Before-Tax	(22,770)
Amortization of prior service cost, Defined Benefit Plans Before-Tax	1,335	1,335	355
Amortization of net actuarial loss, Defined Benefit Plans Before-Tax	27,066	34,381	23,656
Regulatory adjustment, Defined Benefit Plans Before-Tax	(5,584)	(5,646)	140,308
Net current period other comprehensive income (loss), Defined Benefit Plans Before-Tax	47	(449)	(15,988)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Before-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Before-Tax	47
Ending Balance AOCI December 31, 2016, Defined Benefit Plans Before-Tax	(57,613)	(57,660)
Beginning Balance AOCI December 31, 2015, Defined Benefit Plans Tax (Expense) Benefit	21,911
Net actuarial gain/(loss), Tax (Expense) or Benefit	8,652	11,597	65,985
Translation adjustments, Defined Benefit Plans Tax (Expense) Benefit	0
Other comprehensive income before reclassifications, Defined Benefit Plans Tax (Expense) Benefit	8,652
Amortization of prior service cost, Defined Benefit Plans Tax (Expense) Benefit	(507)	(507)	(135)
Amortization of net actuarial loss, Defined Benefit Plans Tax (Expense) Benefit	(10,285)	(13,065)	(8,989)
Regulatory adjustment, Defined Benefit Plans Tax (Expense) Benefit	2,122	2,146	(53,317)
Pension plans other comprehensive income (loss), Tax (Expense) or Benefit	(18)	171	6,075
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans Tax (Expense) Benefit	(18)
Ending Balance AOCI December 31, 2016, Defined Benefit Plans Tax (Expense) Benefit	21,893	21,911
Beginning Balance AOCI December 31, 2015, Defined Benefit Plans After-Tax	(35,749)
Net actuarial gain/(loss), Defined Benefit Plans After-Tax	(14,118)	(18,922)	(107,661)
Translation adjustments, Defined Benefit Plans After-Tax	0
Other comprehensive income before reclassifications, Defined Benefit Plans After-Tax	(14,118)
Amortization of prior service cost, Defined Benefit Plans After-Tax	828	828	220
Amortization of net actuarial (gain)/loss, Defined Benefit Plans After-Tax	16,781	21,316	14,667
Regulatory adjustment, Defined Benefit Plans After-Tax	(3,462)
Net defined benefit pension plans	29	(278)	(9,913)
Less: Translation adjustment attributable to redeemable noncontrolling interest, Defined Benefit Plans After-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Defined Benefit Plans After-Tax	29
Ending Balance AOCI December 31, 2016, Defined Benefit Plans After-Tax	(35,720)	(35,749)
Beginning Balance AOCI December 31, 2015, FSIRS - Before-Tax	(19,344)
Translation adjustments, FSIRS - Before-Tax	0
Other comprehensive income before reclassifications, FSIRS - Before-Tax	0
FSIRS amounts reclassified from AOCI, FSIRS - Before-Tax	3,345	3,344	3,345
Net current period other comprehensive income (loss), FSIRS - Before-Tax	3,345	3,344	3,345
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Before-Tax	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Before-Tax	3,345
Ending Balance AOCI December 31, 2016, FSIRS - Before-Tax	(15,999)	(19,344)
Beginning Balance AOCI December 31, 2015, FSIRS - Tax (Expense) Benefit	7,350
Translation adjustments, FSIRS - Tax (Expense) Benefit	0
Other comprehensive income before reclassifications, FSIRS - Tax (Expense) Benefit	0
FSIRS amounts reclassified from AOCI, FSIRS - Tax (Expense) Benefit	(1,270)	(1,271)	(1,272)
Net current period other comprehensive income (loss), FSIRS - Tax (Expense) Benefit	(1,270)	(1,271)	(1,272)
Less: Translation adjustment attributable to redeemable noncontrolling interest, FSIRS - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - Tax (Expense) Benefit	(1,270)
Ending Balance AOCI December 31, 2016, FSIRS - Tax (Expense) Benefit	6,080	7,350
Beginning Balance AOCI December 31, 2015, FSIRS - After-Tax	(11,994)
Translation adjustments, FSIRS - After-Tax	0
Other comprehensive income before reclassifications	0
FSIRS amounts reclassified from AOCI, FSIRS - After-Tax	2,075	2,073	2,073
Net current period other comprehensive income (loss)	2,075	2,073	2,073
Less: Translation adjustment attributable to redeemable noncontrolling interest	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, FSIRS - After-Tax	2,075
Ending Balance AOCI December 31, 2016, FSIRS - After-Tax	(9,919)	(11,994)
Beginning Balance AOCI December 31, 2015, Foreign Currency Items Before-Tax	(2,525)
Net actuarial gain/(loss)	0
Translation adjustments, Foreign Currency Items Before-Tax	161	(1,954)	(659)
Other comprehensive income before reclassifications	161	(1,954)	(659)
FSIRS amounts reclassified from AOCI, Foreign Currency Items Before-Tax	0
Amortization of prior service cost, Foreign Currency Items Before-Tax	0
Amortization of net actuarial loss, Foreign Currency Items Before-Tax	0
Regulatory adjustment, Foreign Currency Items Before-Tax	0
Net current period other comprehensive income (loss)	161
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency Items Before-Tax	5
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items Before-Tax	156	(1,888)	(637)
Ending Balance AOCI December 31, 2016	(2,369)	(2,525)
Beginning Balance AOCI December 31, 2015, Foreign Currency items - Tax (Expense) Benefit	0
Net actuarial gain/(loss)	0
Translation adjustments, Foreign Currency items - Tax (Expense) Benefit	0	0	0
Other comprehensive income before reclassifications, Foreign Currency items - Tax (Expense) Benefit	0
FSIRS amounts reclassified from AOCI, Foreign Currency items - Tax (Expense) Benefit	0
Amortization of prior service cost ,Foreign Currency items - Tax (Expense) Benefit	0
Amortization of net actuarial loss, Foreign Currency items - Tax (Expense) Benefit	0
Regulatory adjustment, Foreign Currency items - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss), Foreign Currency items - Tax (Expense) Benefit	0	0	0
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - Tax (Expense) Benefit	0
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency items - Tax (Expense) Benefit	0	0	0
Ending Balance AOCI December 31, 2016, Foreign Currency items - Tax (Expense) Benefit	0	0
Beginning Balance AOCI December 31, 2015, Foreign Currency items - After-Tax	(2,525)
Net actuarial gain (loss)	0
Translation adjustments, Foreign Currency items - After-Tax	161	(1,954)	(659)
Other comprehensive income before reclassifications, Foreign Currency items - After-Tax	161	(1,954)	(659)
FSIRS amounts reclassified from AOCI, Foreign Currency items - After-Tax	0
Amortization of prior service cost, Foreign Currency items - After-Tax	0
Amortization of net actuarial loss, Foreign Currency items - After-Tax	0
Regulatory adjustment, Foreign Currency items - After-Tax	0
Net current period other comprehensive income (loss), Foreign Currency items - After-Tax	161
Less: Translation adjustment attributable to redeemable noncontrolling interest, Foreign Currency items - After-Tax	5
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, Foreign Currency Items - After-Tax	156
Ending Balance AOCI December 31, 2016, Foreign Currency items - After-Tax	(2,369)	(2,525)
Beginning Balance AOCI December 31, 2015, AOCI	(50,268)
Net actuarial gain (loss)	(14,118)	(18,922)	(107,661)
Translation adjustments, AOCI	161
Other comprehensive income before reclassifications, AOCI	(13,957)
Amounts reclassified into net income (Notes 5 and 13)	2,075	2,073	2,073
Amortization of prior service cost, AOCI	828	828	220
Amortization of net actuarial loss	16,781	21,316	14,667
Regulatory adjustment, AOCI	(3,462)
Total other comprehensive income (loss), net of tax	2,265	(159)	$ (8,499)
Less: Translation adjustment attributable to redeemable noncontrolling interest, AOCI	5
Net current period other comprehensive income (loss) attributable to Southwest Gas Corporation, AOCI	2,260
Ending Balance AOCI December 31, 2016, AOCI	$ (48,008)	$ (50,268)

Other Comprehensive Income an70

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Rollforward of Accumulated Other Comprehensive Income (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2016
Equity [Abstract]
Other comprehensive income loss, effective income tax rate	38.00%

Other Comprehensive Income an71

Other Comprehensive Income and Accumulated Other Comprehensive Income ("AOCI") - Amount Recognized Before Income Tax Associated with Defined Benefit Plans in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: amount recognized in regulatory assets	$ 477,743	$ 488,257
Recognized in AOCI	(57,613)	(57,660)
Defined Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial (loss) gain	(430,973)	(435,269)
Prior service cost	(5,703)	(7,038)
Less: amount recognized in regulatory assets	379,063	384,647
Recognized in AOCI	$ (57,613)	$ (57,660)

Common Stock - Additional Infor

Common Stock - Additional Information (Detail) - USD ($)	Mar. 10, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Value of common stock	$ 100,000,000
Net proceeds to Company		$ 472,000	$ 35,396,000	$ 405,000
Common stock issued through Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan		105,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent		0			645,225
Net proceeds to Company					$ 35,167,584
Agent commissions					$ 355,228

Long-Term Debt - Schedule of Ca

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	$ (50,101)		$ (19,475)
Long-term debt, less current maturities	1,549,983		1,551,204
Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Carrying Amount [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs	(516)		(692)
Total long-term debt	1,600,084		1,570,679
Long-term Debt, Current and Noncurrent Abstract
Less: current maturities	(50,101)		(19,475)
Long-term debt, less current maturities	1,549,983		1,551,204
Total long-term debt	1,600,084		1,570,679
Carrying Amount [Member] | Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	41,185		60,627
Carrying Amount [Member] | IDRBs [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	106,184		111,879
Unsecured debt	197,511		320,909
Carrying Amount [Member] | Centuri Term Loan Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	106,700		112,571
Carrying Amount [Member] | Centuri Other Debt Obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	52,635		25,901
Carrying Amount [Member] | Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(9,931)		(6,137)
Notes payable	1,197,569		901,363
Carrying Amount [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable	125,000		125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable	250,000		250,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Notes payable	300,000	$ 300,000	0
Carrying Amount [Member] | Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable	75,000		75,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	25,000		25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	7,500		7,500
Carrying Amount [Member] | Debentures [Member] | Revolving Credit Facility and Commercial Paper [Member]
Debt Instrument [Line Items]
Line of credit facility	5,000		150,000
Carrying Amount [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000		50,000
Carrying Amount [Member] | Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs	(2,489)		(3,946)
Carrying Amount [Member] | Fixed-Rate Bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	0		100,000
Carrying Amount [Member] | Fixed-Rate Bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	0		24,855
Market Value [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	106,819		112,665
Centuri other debt obligations	52,840		26,059
Market Value [Member] | Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Centuri term loan/secured revolving credit facility	41,292		60,724
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	129,703		130,273
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	149,734		141,581
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	254,900		253,600
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	266,793		251,483
Market Value [Member] | Debentures [Member] | Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	283,029		0
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	94,691		97,035
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	25,040		26,253
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	29,290		29,855
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,905		31,890
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	8,769		8,684
Market Value [Member] | Debentures [Member] | Revolving Credit Facility and Commercial Paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	5,000		150,000
Market Value [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Market Value [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Market Value [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	50,000		50,000
Market Value [Member] | Fixed-Rate Bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	0		100,452
Market Value [Member] | Fixed-Rate Bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt, fair value	$ 0		$ 25,130

Long-Term Debt - Schedule of 74

Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Parenthetical) (Detail)	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Debentures [Member] | Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		4.45%	4.45%
Debt instrument due date		Dec. 1, 2020	Dec. 1, 2020
Debentures [Member] | Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.10%	6.10%
Debt instrument due date		Feb. 15, 2041	Feb. 15, 2041
Debentures [Member] | Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		3.875%	3.875%
Debt instrument due date		Apr. 1, 2022	Apr. 1, 2022
Debentures [Member] | Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		4.875%	4.875%
Debt instrument due date		Oct. 1, 2043	Oct. 1, 2043
Debentures [Member] | Notes, 3.8%, due 2046 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	3.80%	3.80%
Debt instrument due date	Sep. 30, 2046	Sep. 30, 2046
Debentures [Member] | 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		8.00%	8.00%
Debt instrument due date		Aug. 1, 2026	Aug. 1, 2026
Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.59%	7.59%
Debt instrument due date		Jan. 17, 2017	Jan. 17, 2017
Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.78%	7.78%
Debt instrument due date		Feb. 3, 2022	Feb. 3, 2022
Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		7.92%	7.92%
Debt instrument due date		Jun. 24, 2027	Jun. 24, 2027
Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		6.76%	6.76%
Debt instrument due date		Sep. 24, 2027	Sep. 24, 2027
Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Dec. 1, 2028	Dec. 1, 2028
Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Mar. 1, 2038	Mar. 1, 2038
Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Mar. 1, 2038	Mar. 1, 2038
Fixed-Rate Bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate			4.85%
Debt instrument due date			Oct. 1, 2035
Fixed-Rate Bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate			4.75%
Debt instrument due date			Sep. 1, 2036
Market Value [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Debt instrument due date		Dec. 1, 2039	Dec. 1, 2039

Long-Term Debt - Additional Inf

Long-Term Debt - Additional Information (Detail) - USD ($)	1 Months Ended				3 Months Ended		12 Months Ended
	Jan. 31, 2017	Sep. 30, 2016	Jul. 31, 2016	Feb. 29, 2016	Sep. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Line of credit designated for working capital purposes							$ 150,000,000
Minimum current borrowing usage					$ 0
Maximum current borrowing usage							$ 230,000,000
Debt instrument covenant description							Certain debt instruments contain securities ratings covenants that, if set in motion, would increase financing costs. Certain debt instruments also have leverage ratio caps and minimum net worth requirements. At December 31, 2016, the Company is in compliance with all of its covenants.
Issuance of additional debt							$ 2,300,000,000
Minimum net worth							1,100,000,000
Short-Term Credit Facility [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility							$ 0	$ 18,000,000
Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Minimum current borrowing usage				$ 36,200,000
Maximum current borrowing usage			$ 83,200,000
Secured Revolving Credit and Term Loan Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commitment fee							0.15%
Secured Revolving Credit and Term Loan Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Commitment fee							0.40%
Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Effective interest rate							2.63%
Debentures [Member] | Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable							$ 1,197,569,000	$ 901,363,000
Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Debt instrument covenant description							Certain Centuri debt instruments also have leverage ratio caps and fixed charge ratio coverage requirements. At December 31, 2016, Centuri is in compliance with all of its covenants.
Issuance of additional debt							$ 145,000,000
Equity cushion relating to minimum fixed charge ratio coverage requirement							21,000,000
Centuri Construction Group Inc [Member] | Secured Revolving Credit and Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity							$ 300,000,000
Credit facility expiration date							Oct. 31, 2019
Debt secured by assets							$ 445,000,000
Centuri Construction Group Inc [Member] | Centuri Secured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity							$ 150,000,000
LIBOR [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Applicable margin							1.00%
LIBOR [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Applicable margin							2.25%
Canadian Dealer Offered Rate [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Applicable margin							1.00%
Canadian Dealer Offered Rate [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Applicable margin							2.25%
Alternate Base Rate or Canadian Base Rate [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Applicable margin							0.00%
Alternate Base Rate or Canadian Base Rate [Member] | Secured Revolving Credit and Term Loan Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Applicable margin							1.25%
4.85% 2005 Series A, due 2035 [Member] | Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate								4.85%
Debt instrument due date								Oct. 1, 2035
4.85% 2005 Series A, due 2035 [Member] | Fixed-Rate Bonds [Member] | Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt							$ 0	$ 100,000,000
4.85% 2005 Series A, due 2035 [Member] | Early Redemption of Debt [Member] | Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt			$ 100,000,000
Debt instrument, original debt due date of debt year			2,035
Debt instrument interest rate			4.85%
4.75% 2006 Series A, due 2036 [Member] | Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Debt instrument interest rate								4.75%
Debt instrument due date								Sep. 1, 2036
4.75% 2006 Series A, due 2036 [Member] | Fixed-Rate Bonds [Member] | Carrying Amount [Member]
Debt Instrument [Line Items]
Unsecured debt							$ 0	$ 24,855,000
4.75% 2006 Series A, due 2036 [Member] | Early Redemption of Debt [Member] | Fixed-Rate Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt		$ 24,900,000			$ 24,900,000
Debt instrument, original debt due date of debt year		2,036
Debt instrument interest rate		4.75%			4.75%
Southwest Gas Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity						$ 300,000,000
Line of credit designated as long term debt						150,000,000
Line of credit designated for working capital purposes						$ 150,000,000
Credit facility expiration date						Mar. 1, 2021
Effective interest rate							5.21%
Southwest Gas Credit Facility [Member] | Long-term Debt [Member]
Debt Instrument [Line Items]
Borrowings outstanding under facility							$ 5,000,000
Southwest Gas Credit Facility [Member] | Before Amendment [Member]
Debt Instrument [Line Items]
Previous credit facility expiration date						Mar. 1, 2020
Southwest Gas Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Applicable margin							1.00%
Southwest Gas Credit Facility [Member] | Alternative Base Rate [Member]
Debt Instrument [Line Items]
Applicable margin							0.00%
Commercial Paper Program [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity							$ 50,000,000
Borrowings outstanding under facility							$ 0
Notes, 3.8%, due 2046 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate		3.80%			3.80%		3.80%
Debt instrument due date		Sep. 30, 2046					Sep. 30, 2046
Debt instrument discount percent at issuance		0.302%			0.302%
Notes, 3.8%, due 2046 [Member] | Debentures [Member] | Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable		$ 300,000,000			$ 300,000,000		$ 300,000,000	$ 0
Centuri Term Loan Facility [Member] | Centuri Construction Group Inc [Member]
Debt Instrument [Line Items]
Credit facility, face amount							$ 150,000,000
Medium-term notes, 7.59% series, due 2017 [Member] | Debentures [Member]
Debt Instrument [Line Items]
Debt instrument interest rate							7.59%	7.59%
Debt instrument due date							Jan. 17, 2017	Jan. 17, 2017
Medium-term notes, 7.59% series, due 2017 [Member] | Debentures [Member] | Carrying Amount [Member]
Debt Instrument [Line Items]
Notes payable							$ 25,000,000	$ 25,000,000
Medium-term notes, 7.59% series, due 2017 [Member] | Subsequent Event [Member]
Debt Instrument [Line Items]
Debt instrument interest rate	7.59%
Repayments of medium term notes	$ 25,000,000

Long-Term Debt - Summary of Eff

Long-Term Debt - Summary of Effective Interest Rates on Variable-Rate IDRBs (Detail)	Dec. 31, 2016	Dec. 31, 2015
2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Effective interest rates	1.47%	0.87%
2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Effective interest rates	1.53%	0.87%
2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Effective interest rates	1.43%	0.75%
Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Effective interest rates	1.51%	0.81%

Long-Term Debt - Estimated Matu

Long-Term Debt - Estimated Maturities of Long-Term Debt (Detail) $ in Thousands	Dec. 31, 2016USD ($)
Debt Disclosure [Abstract]
2,017	$ 50,101
2,018	24,082
2,019	134,534
2,020	134,452
2,021	$ 7,815

Short-Term Debt - Additional In

Short-Term Debt - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Short-term Debt [Line Items]
Line of credit designated for working capital purposes	$ 150,000,000
Short-term borrowings outstanding on the credit facility	0	$ 18,000,000
March 2021 [Member]
Short-term Debt [Line Items]
Credit facility maximum borrowing capacity	$ 300,000,000
Credit facility expiration date	Mar. 31, 2021

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	Dec. 31, 2016USD ($)
Commitments and Contingencies Disclosure [Abstract]
Self-insured retention amount associated with general liability claims	$ 1
Additional self-insured retention amount of general liability	$ 4

Pension and Other Postretirem80

Pension and Other Postretirement Benefits - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2016USD ($)	Sep. 30, 2016Participants	Dec. 31, 2016USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Maximum contribution of employer			3.50%
Deferment of annual cash compensation			100.00%
Matching contribution of employer			3.50%
Deferred compensation payments on retirement			Upon retirement, payments of compensation deferred, plus interest, are made in equal monthly installments over 10, 15, or 20 years, as elected by the participant.
Additional Deferred Compensation Payment Options, period of payment, years			5 years
Deferred compensation, percentage of multiple interest rate			150.00%
Description of specific offer to select employees			About one-half of the approximate 800 participants subject to the offer accepted the offer, resulting in an approximate $30 million payment from pension assets paid in the fourth quarter of 2016.
Number of employees offered a lump-sum payout | Participants		800
Lump sum pension payment to be paid	$ 30
Increments of changes to the discount rate, basis points			0.25%
Future estimated funding amount			$ 39
Maximum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption			7.00%
Minimum [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Per capita cost of covered health care benefits medical rate trend assumption			4.50%
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Future estimated funding amount			$ 36

Pension and Other Postretirem81

Pension and Other Postretirement Benefits - Cost of Retirement Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Employee Investment Plan cost	$ 4,976	$ 5,072	$ 4,816

Pension and Other Postretirem82

Pension and Other Postretirement Benefits - Schedule of Percentage Range of Target Portfolio (Detail)	12 Months Ended
	Dec. 31, 2016
Minimum [Member] | Equity Securities [Member]
Percentage of Investment in Retirement Plan [Line Items]
Percentage range of investment	63.00%
Minimum [Member] | Debt Securities [Member]
Percentage of Investment in Retirement Plan [Line Items]
Percentage range of investment	33.00%
Maximum [Member] | Equity Securities [Member]
Percentage of Investment in Retirement Plan [Line Items]
Percentage range of investment	67.00%
Maximum [Member] | Debt Securities [Member]
Percentage of Investment in Retirement Plan [Line Items]
Percentage range of investment	37.00%
Maximum [Member] | Other Up To [Member]
Percentage of Investment in Retirement Plan [Line Items]
Percentage range of investment	1.00%

Pension and Other Postretirem83

Pension and Other Postretirement Benefits - Schedule of Assumptions Used (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]
Discount rate	4.50%	4.50%
Weighted-average rate of compensation increase	3.25%	3.25%
Asset return assumption	7.00%	7.25%

Pension and Other Postretirem84

Pension and Other Postretirement Benefits - Schedule of Amounts Recognized in Balance Sheet and Income Statement (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Qualified Retirement Plan [Member]
Change in benefit obligations
Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 1,044,817	$ 1,060,240
Service cost	22,833	25,123	$ 21,360
Interest cost	46,027	44,229	43,440
Actuarial loss (gain)	8,550	(44,553)
Benefits paid	(73,874)	(40,222)
Benefit obligation at end of year (PBO/PBO/APBO)	1,048,353	1,044,817	1,060,240
Change in plan assets
Market value of plan assets, beginning balance	736,880	754,796
Actual return on plan assets	39,956	(13,694)
Employer contributions	36,000	36,000
Benefits paid	(73,874)	(40,222)
Market value of plan assets, ending balance	738,962	736,880	754,796
Funded status at year end	$ (309,391)	$ (307,937)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50%	4.50%
Weighted-average rate of compensation increase	3.25%	3.25%
SERP [Member]
Change in benefit obligations
Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 42,720	$ 41,176
Service cost	331	320	292
Interest cost	1,859	1,695	1,745
Actuarial loss (gain)	1,347	2,322
Benefits paid	(2,946)	(2,793)
Benefit obligation at end of year (PBO/PBO/APBO)	43,311	42,720	41,176
Change in plan assets
Market value of plan assets, beginning balance	0	0
Actual return on plan assets	0	0
Employer contributions	2,946	2,793
Benefits paid	(2,946)	(2,793)
Market value of plan assets, ending balance	0	0	0
Funded status at year end	$ (43,311)	$ (42,720)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50%	4.50%
Weighted-average rate of compensation increase	3.25%	3.25%
PBOP [Member]
Change in benefit obligations
Change in benefit obligations Benefit obligation for service rendered to date at beginning of year (PBO/PBO/APBO)	$ 72,632	$ 72,202
Service cost	1,499	1,641	1,101
Interest cost	3,180	2,999	2,829
Actuarial loss (gain)	(2,060)	(3,251)
Benefits paid	(1,386)	(959)
Benefit obligation at end of year (PBO/PBO/APBO)	73,865	72,632	72,202
Change in plan assets
Market value of plan assets, beginning balance	43,584	44,892
Actual return on plan assets	4,818	(1,034)
Employer contributions	0	0
Benefits paid	(289)	(274)
Market value of plan assets, ending balance	48,113	43,584	$ 44,892
Funded status at year end	$ (25,752)	$ (29,048)
Weighted-average assumptions (benefit obligation)
Discount rate	4.50%	4.50%

Pension and Other Postretirem85

Pension and Other Postretirement Benefits - Schedule of Accumulated Benefit Obligation (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 939,002	$ 922,992
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	$ 40,852	$ 39,270

Pension and Other Postretirem86

Pension and Other Postretirement Benefits - Schedule of Expected Benefit Payments (Detail) $ in Millions	Dec. 31, 2016USD ($)
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
2,017	$ 48.6
2,018	50.1
2,019	51.5
2,020	53.2
2,021	55.1
2022-2026	294.2
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
2,017	4.1
2,018	4.3
2,019	4.4
2,020	4.5
2,021	4.5
2022-2026	20.6
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
2,017	2.9
2,018	2.9
2,019	2.9
2,020	2.9
2,021	2.9
2022-2026	$ 14.4

Pension and Other Postretirem87

Pension and Other Postretirement Benefits - Schedule of Net Periodic Benefit Cost and Weighted-Average Assumptions (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 44,423	$ 50,451	$ 38,172
Weighted-average assumptions (net benefit cost)
Discount rate	4.50%	4.50%
Expected return on plan assets	7.00%	7.25%
Weighted-average rate of compensation increase	3.25%	3.25%
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 22,833	$ 25,123	21,360
Interest cost	46,027	44,229	43,440
Expected return on plan assets	(56,558)	(57,808)	(53,342)
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	25,266	32,743	22,873
Net periodic benefit cost	$ 37,568	$ 44,287	$ 34,331
Weighted-average assumptions (net benefit cost)
Discount rate	4.50%	4.25%	5.00%
Expected return on plan assets	7.25%	7.75%	7.75%
Weighted-average rate of compensation increase	3.25%	2.75%	3.25%
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 331	$ 320	$ 292
Interest cost	1,859	1,695	1,745
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	1,383	1,293	783
Net periodic benefit cost	$ 3,573	$ 3,308	$ 2,820
Weighted-average assumptions (net benefit cost)
Discount rate	4.50%	4.25%	5.00%
Weighted-average rate of compensation increase	3.25%	2.75%	3.25%
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,499	$ 1,641	$ 1,101
Interest cost	3,180	2,999	2,829
Expected return on plan assets	(3,149)	(3,464)	(3,264)
Amortization of prior service cost	1,335	1,335	355
Amortization of net actuarial loss	417	345	0
Net periodic benefit cost	$ 3,282	$ 2,856	$ 1,021
Weighted-average assumptions (net benefit cost)
Discount rate	4.50%	4.25%	5.00%
Expected return on plan assets	7.25%	7.75%	7.75%

Pension and Other Postretirem88

Pension and Other Postretirement Benefits - Schedule of Other Changes in Plan Assets and Benefit Obligations Recognized in Net Periodic Benefit Cost and Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	$ 22,770	$ 30,519	$ 173,646
Amortization of prior service cost	(1,335)	(1,335)	(355)
Amortization of net actuarial loss	(27,066)	(34,381)	(23,656)
Prior service cost	0	0	6,661
Regulatory adjustment	5,584	5,646	(140,308)
Recognized in other comprehensive (income) loss	(47)	449	15,988
Net periodic benefit costs recognized in net income	44,423	50,451	38,172
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	44,376	50,900	54,160
Qualified Retirement Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	25,153	26,949	163,215
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	(25,266)	(32,743)	(22,872)
Prior service cost	0	0	0
Regulatory adjustment	102	5,214	(129,031)
Recognized in other comprehensive (income) loss	(11)	(580)	11,312
Net periodic benefit costs recognized in net income	37,568	44,287	34,331
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	37,557	43,707	45,643
SERP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	1,347	2,322	5,460
Amortization of prior service cost	0	0	0
Amortization of net actuarial loss	(1,383)	(1,293)	(784)
Prior service cost	0	0	0
Regulatory adjustment	0	0	0
Recognized in other comprehensive (income) loss	(36)	1,029	4,676
Net periodic benefit costs recognized in net income	3,573	3,308	2,820
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	3,537	4,337	7,496
PBOP [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Net actuarial loss (gain)	(3,730)	1,248	4,971
Amortization of prior service cost	(1,335)	(1,335)	(355)
Amortization of net actuarial loss	(417)	(345)	0
Prior service cost	0	0	6,661
Regulatory adjustment	5,482	432	(11,277)
Recognized in other comprehensive (income) loss	0	0	0
Net periodic benefit costs recognized in net income	3,282	2,856	1,021
Total of amount recognized in net periodic benefit cost and other comprehensive (income) loss	$ 3,282	$ 2,856	$ 1,021

Pension and Other Postretirem89

Pension and Other Postretirement Benefits - Schedule of Fair Value of Plan Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets, including insurance contracts	$ 787,074	$ 783,323
Assets at Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	783,626	521,638
Commingled Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	257,966
Insurance Company General Account Contracts (Member]
Defined Benefit Plan Disclosure [Line Items]
Contract value of insurance contracts	3,448	3,719
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,922	364,610
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Agriculture [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	7,230
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Capital Equipment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	549
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Chemicals/Materials [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3,402
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Consumer Goods [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	42,256
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Energy and Mining [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	11,395
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Finance/Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	30,849
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Healthcare [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	39,059
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Information Technology [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	30,099
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Services [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	12,708
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Telecommunications/Internet/Media [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	26,653
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	9,312
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Real Estate Investment Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	5,159
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,922	111,468
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Government Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	34,478
Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Futures Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	(7)
Level 2 - Significant Other Observable Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	758,704	157,028
Level 2 - Significant Other Observable Inputs [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	31,917
Level 2 - Significant Other Observable Inputs [Member] | Real Estate Investment Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2,007
Level 2 - Significant Other Observable Inputs [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	299,808	0
Level 2 - Significant Other Observable Inputs [Member] | Large and Medium Capitalization [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	125,253	0
Level 2 - Significant Other Observable Inputs [Member] | Small Capitalization [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	26,763	0
Level 2 - Significant Other Observable Inputs [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46,733	0
Level 2 - Significant Other Observable Inputs [Member] | U S Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	166,152	0
Level 2 - Significant Other Observable Inputs [Member] | US Debt Market Long Duration [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	79,781	0
Level 2 - Significant Other Observable Inputs [Member] | US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,937	0
Level 2 - Significant Other Observable Inputs [Member] | Pooled Funds and Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,105	15,604
Level 2 - Significant Other Observable Inputs [Member] | Government Fixed Income and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	172	51,046
Level 2 - Significant Other Observable Inputs [Member] | Asset-Backed and Mortgage-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	24,243
Level 2 - Significant Other Observable Inputs [Member] | Banking [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	21,478
Level 2 - Significant Other Observable Inputs [Member] | Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	5,042
Level 2 - Significant Other Observable Inputs [Member] | Utilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3,940
Level 2 - Significant Other Observable Inputs [Member] | State and Local Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	978
Level 2 - Significant Other Observable Inputs [Member] | Preferred Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	571
Level 2 - Significant Other Observable Inputs [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	202
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	738,962	736,880	$ 754,796
Fair value of plan assets, including insurance contracts	738,962	739,684
Qualified Retirement Plan [Member] | Assets at Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	735,514	485,454
Qualified Retirement Plan [Member] | Commingled Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	250,511
Qualified Retirement Plan [Member] | Insurance Company General Account Contracts (Member]
Defined Benefit Plan Disclosure [Line Items]
Contract value of insurance contracts	3,448	3,719
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	333,310
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Agriculture [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	7,021
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Capital Equipment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	533
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Chemicals/Materials [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3,304
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Consumer Goods [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	41,035
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Energy and Mining [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	11,066
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Finance/Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	29,957
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Healthcare [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	37,930
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Information Technology [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	29,229
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Services [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	12,341
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Telecommunications/Internet/Media [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	25,883
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	9,043
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Real Estate Investment Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	5,010
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	87,483
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Government Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	33,482
Qualified Retirement Plan [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Futures Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	(7)
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	735,514	152,144
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	30,995
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Real Estate Investment Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1,949
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	290,668	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Large and Medium Capitalization [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	121,434	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Small Capitalization [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25,947	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45,309	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | U S Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	161,086	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | US Debt Market Long Duration [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	77,349	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,665	0
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Pooled Funds and Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,889	14,808
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Government Fixed Income and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	167	49,571
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Asset-Backed and Mortgage-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	23,542
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Banking [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	20,857
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	4,896
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Utilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3,826
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | State and Local Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	950
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Preferred Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	554
Qualified Retirement Plan [Member] | Level 2 - Significant Other Observable Inputs [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	196
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	48,113	43,584	$ 44,892
Fair value of plan assets, including insurance contracts	48,112	43,639
PBOP [Member] | Assets at Fair Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	48,112	36,184
PBOP [Member] | Commingled Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	7,455
PBOP [Member] | Insurance Company General Account Contracts (Member]
Defined Benefit Plan Disclosure [Line Items]
Contract value of insurance contracts	0	0
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,922	31,300
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Agriculture [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	209
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Capital Equipment [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	16
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Chemicals/Materials [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	98
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Consumer Goods [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1,221
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Energy and Mining [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	329
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Finance/Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	892
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Healthcare [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1,129
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Information Technology [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	870
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Services [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	367
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Telecommunications/Internet/Media [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	770
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	269
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Real Estate Investment Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	149
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,922	23,985
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Government Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	996
PBOP [Member] | Level 1 - Quoted Prices in Active Markets for Identical Financial Assets [Member] | Futures Contracts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23,190	4,884
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	922
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Real Estate Investment Trusts [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	58
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | International [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,140	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Large and Medium Capitalization [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,819	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Small Capitalization [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	816	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,424	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | U S Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,066	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | US Debt Market Long Duration [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,432	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	272	0
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Pooled Funds and Mutual Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	216	796
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Government Fixed Income and Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	1,475
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Asset-Backed and Mortgage-Backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	701
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Banking [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	621
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Insurance [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	146
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Utilities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	114
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | State and Local Obligations [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	28
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Preferred Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	17
PBOP [Member] | Level 2 - Significant Other Observable Inputs [Member] | Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 6

Pension and Other Postretirem90

Pension and Other Postretirement Benefits - Schedule of Fair Value of Plan Assets (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Amounts not included in funded status table		$ 2,859,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Redemption request notice to prior period	1 day
Impact fee on redemptions and subscriptions of $5 million or more		5,000,000
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Redemption request notice to prior period	30 days
Qualified Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Amounts not included in funded status table		2,803,000
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Amounts not included in funded status table		$ 56,000

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock-Based Plan Compensation Expense, Including Cash Award (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock-based compensation plan expense, net of related tax benefits	$ 7,185	$ 7,278	$ 8,130
Stock-based compensation plan related tax benefits	$ 4,404	$ 4,461	$ 4,983

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016USD ($)	Dec. 31, 2015$ / shares	Dec. 31, 2014$ / shares
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Options, maximum term, years	10 years
Cash received from exercise of stock options	$ 735,000
Tax benefit realized from exercise of stock options	$ 205,000
Percentage of restricted stock/units vested during first year	0.40
Percentage of restricted stock/units vested during second and third years	0.30
Weighted average grant date fair value of performance shares and restricted stock/units granted | $ / shares		$ 63.09	$ 53.73
Total compensation cost related to nonvested performance shares and restricted stock/units not yet recognized	$ 3,300,000

Stock-Based Compensation - Sc93

Stock-Based Compensation - Schedule of Stock Options Activity (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Number of options, Outstanding at the beginning of the year	17	36	52
Number of options, Exercised during the year	(17)	(19)	(16)
Number of options, Forfeited or expired during the year	0	0	0
Number of options, Outstanding and exercisable at year end	0	17	36
Weighted-average exercise price, Outstanding at the beginning of the year	$ 31.64	$ 28.97	$ 27.57
Weighted-average exercise price, Exercised during the year	31.64	26.69	24.31
Weighted-average exercise price, Forfeited or expired during the year	$ 0	0	0
Weighted-average exercise price, Outstanding and exercisable at year end		$ 31.64	$ 28.97

Stock-Based Compensation - Sc94

Stock-Based Compensation - Schedule of Aggregate Intrinsic Value of Outstanding and Exercisable Options (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Outstanding and exercisable	$ 0	$ 394	$ 1,194
Exercised	$ 554	$ 590	$ 451

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Market Prices of Common Stock (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Market value of Company stock	$ 76.62	$ 55.16	$ 61.81

Stock-Based Compensation - Sc96

Stock-Based Compensation - Schedule of Nonvested Performance and Restricted Stock Unit Plans (Detail) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average grant date fair value, Granted		$ 63.09	$ 53.73
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested/unissued, Number of Units, at beginning of year	197
Nonvested, Number of Units, Granted	44
Nonvested, Number of Units, Dividends	5
Nonvested, Number of Units, Forfeited or expired	0
Nonvested, Number of Units, Vested and issued	(78)
Nonvested/unissued, Number of Units, Ending balance	168	197
Weighted-average grant date fair value, Beginning balance	$ 50.63
Weighted-average grant date fair value, Granted	59.05
Weighted-average grant date fair value, Dividends	0
Weighted-average grant date fair value, Forfeited or expired	0
Weighted-average grant date fair value, Vested and issued	41.82
Weighted-average grant date fair value, Ending balance	$ 55.62	$ 50.63
Restricted Stock/Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Nonvested/unissued, Number of Units, at beginning of year	228
Nonvested, Number of Units, Granted	73
Nonvested, Number of Units, Dividends	6
Nonvested, Number of Units, Forfeited or expired	0
Nonvested, Number of Units, Vested and issued	(45)
Nonvested/unissued, Number of Units, Ending balance	262	228
Weighted-average grant date fair value, Beginning balance	$ 44.36
Weighted-average grant date fair value, Granted	60.39
Weighted-average grant date fair value, Dividends	0
Weighted-average grant date fair value, Forfeited or expired	0
Weighted-average grant date fair value, Vested and issued	51.98
Weighted-average grant date fair value, Ending balance	$ 46.41	$ 44.36

Income Taxes - Summary of Incom

Income Taxes - Summary of Income Before Taxes and Noncontrolling Interest for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax [Line Items]
Total income before income taxes	$ 231,523	$ 219,332	$ 219,521
Foreign [Member]
Income Tax [Line Items]
Total income before income taxes	12,713	(2,328)	(1,950)
United States [Member]
Income Tax [Line Items]
Total income before income taxes	$ 218,810	$ 221,660	$ 221,471

Income Taxes - Summary of Inc98

Income Taxes - Summary of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 541	$ 21,321	$ 1,739
State	5,748	9,899	5,073
Foreign	4,298	650	2,193
Income tax expense (benefit), Current	10,587	31,870	9,005
Deferred:
Federal	68,270	51,132	71,439
State	140	(2,574)	614
Foreign	(529)	(526)	(2,685)
Total deferred income tax expense	67,881	48,032	69,368
Total income tax expense	$ 78,468	$ 79,902	$ 78,373

Income Taxes - Significant Comp

Income Taxes - Significant Components of Deferred Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred federal and state:
Total deferred federal and state	$ 68,719	$ 48,893	$ 70,229
Deferred ITC, net	(838)	(861)	(861)
Total deferred income tax expense	67,881	48,032	69,368
Property-related Items [Member]
Deferred federal and state:
Total deferred federal and state	76,217	65,931	52,814
Purchased Gas Cost Adjustments [Member]
Deferred federal and state:
Total deferred federal and state	361	(32,993)	15,049
Employee Benefits [Member]
Deferred federal and state:
Total deferred federal and state	(1,327)	623	109
All Other Deferred [Member]
Deferred federal and state:
Total deferred federal and state	$ (6,532)	$ 15,332	$ 2,257

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of U.S Federal Statutory Rate to Consolidated Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	35.00%	35.00%	35.00%
Net state taxes	1.40%	1.80%	1.90%
Property-related items	0.00%	0.10%	0.10%
Tax credits	(0.40%)	(0.40%)	(0.50%)
Company owned life insurance	(1.20%)	0.10%	(1.00%)
All other differences	(0.90%)	(0.20%)	0.20%
Consolidated effective income tax rate	33.90%	36.40%	35.70%

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Deferred income taxes for future amortization of ITC	$ 1,094	$ 1,614
Employee benefits	38,231	36,923
Alternative minimum tax credit	4,827	4,809
Net operating losses and credits	1,204	868
Interest rate swap	6,080	7,351
Other	18,415	24,636
Valuation allowance	(495)	(499)
Deferred tax assets, total	69,356	75,702
Deferred tax liabilities:
Property-related items, including accelerated depreciation	872,136	794,850
Regulatory balancing accounts	1,104	743
Unamortized ITC	1,710	2,549
Debt-related costs	5,712	5,497
Intangibles	8,803	9,547
Other	19,256	31,533
Deferred tax liabilities, total	908,721	844,719
Net noncurrent deferred tax liabilities	$ 839,365	$ 769,017

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Line Items]
Undistributed foreign earnings	$ 5,000,000
Unrecognized tax benefits that, if recognized, would affect the effective tax rate	935,000
Significant increases or decreases in unrecognized tax benefit within the next 12 months	0
Tax-related interest income	$ 0	$ 0	$ 0
Minimum [Member]
Income Tax Disclosure [Line Items]
Amount of benefit being recognized in financial statements upon ultimate settlement, Percentage	50.00%
United States [Member]
Income Tax Disclosure [Line Items]
Federal net capital loss carryforward	$ 278,000
Net capital loss carryforward expiration year	2,017
Canada [Member]
Income Tax Disclosure [Line Items]
Federal net operating loss carryforward	$ 4,500,000
Net operating loss carryforward expiration year	2,032

Income Taxes - Reconciliatio103

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits at beginning of year	$ 296	$ 305
Gross increases - tax positions in prior period	897	0
Gross decreases - tax positions in prior period	0	(9)
Gross increases - current period tax positions	38	0
Gross decreases - current period tax positions	0	0
Settlements	0	0
Lapse in statute of limitations	0	0
Unrecognized tax benefits at end of year	$ 1,231	$ 296

Derivatives and Fair Value M104

Derivatives and Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 2,075,000	$ 2,073,000	$ 2,073,000
Derivative instrument loss at settlement amortization period	10 years
Cash Flow Hedging [Member]
Derivative [Line Items]
Gains (losses) recognized in income or other comprehensive income for derivative designated cash flow hedges	$ 0
Minimum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	Jan. 31, 2017
Maximum [Member]
Derivative [Line Items]
Maturities of natural gas swaps	Mar. 31, 2019
Arizona [Member]
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%
California [Member]
Derivative [Line Items]
Natural gas portfolios, maximum % rate	25.00%

Derivatives and Fair Value M105

Derivatives and Fair Value Measurements - Notional Amounts under Swaps Contracts (Detail) - MMBTU	Dec. 31, 2016	Dec. 31, 2015
Offsetting [Abstract]
Contract notional amounts	10,543,000	7,407,000

Derivatives and Fair Value M106

Derivatives and Fair Value Measurements - Amount of Gain or Losses Recognized in Income on Derivatives (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ 0	$ 0	$ 0
Cash Flow Hedging [Member] | Net Cost of Gas Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	5,006	(7,598)	(2,363)
Regulatory Deferral Accounting Treatment [Member] | Cash Flow Hedging [Member] | Net Cost of Gas Sold [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain or (loss) recognized in income on derivative	$ (5,006)	$ 7,598	$ 2,363

Derivatives and Fair Value M107

Derivatives and Fair Value Measurements - Fair Values of Swaps in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 4,450	$ 4
Liability derivatives not designated as hedging instruments	(73)	(5,490)
Net total not designated as hedging instruments	4,377	(5,486)
Swaps [Member] | Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	899
Liability derivatives not designated as hedging instruments	(54)
Net total not designated as hedging instruments	845
Swaps [Member] | Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	3,551
Liability derivatives not designated as hedging instruments	(19)
Net total not designated as hedging instruments	$ 3,532
Swaps [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		0
Liability derivatives not designated as hedging instruments		(4,267)
Net total not designated as hedging instruments		(4,267)
Swaps [Member] | Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		4
Liability derivatives not designated as hedging instruments		(1,223)
Net total not designated as hedging instruments		$ (1,219)

Derivatives and Fair Value M108

Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 5,583	$ 7,537	$ 829
Received from counterparties	$ 726	$ 0	$ 4,713

Derivatives and Fair Value M109

Derivatives and Fair Value Measurements - Regulatory Assets/Liabilities Offsetting Derivatives at Fair Value in Condensed Consolidated Balance Sheets (Detail) - Swaps [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ (845)
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Net Total	$ (3,532)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ 4,267
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Net Total		$ 1,219

Derivatives and Fair Value M110

Derivatives and Fair Value Measurements - Significant Other Observable Inputs (Detail) - Level 2 - Significant Other Observable Inputs [Member] - Swaps [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Net Assets (Liabilities)	$ 4,377	$ (5,486)
Prepaids and Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	3,532	0
Deferred Charges and Other Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	845	0
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	0	(4,267)
Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Derivative Liabilities	$ 0	$ (1,219)

Segment Information - Additiona

Segment Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016
Net Assets, Geographic Area [Member] | Geographic Concentration Risk [Member] | United States [Member] | Minimum [Member]
Segment Reporting Information [Line Items]
Long-lived assets	99.00%

Segment Information - Accounts

Segment Information - Accounts Receivable for Services (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Centuri Construction Group Inc [Member]
Segment Reporting Information [Line Items]
Accounts receivable for Centuri services	$ 10,585	$ 10,006

Segment Information - Schedule

Segment Information - Schedule of Revenues by Geographic Area (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total Revenues	$ 641,525	$ 539,969	$ 547,748	$ 731,248	$ 685,405	$ 505,396	$ 538,604	$ 734,220	$ 627,683	$ 432,475	$ 453,153	$ 608,396	$ 2,460,490	$ 2,463,625	$ 2,121,707
United States [Member]
Segment Reporting Information [Line Items]
Total Revenues													2,256,600	2,289,133	2,069,513
Canada [Member]
Segment Reporting Information [Line Items]
Total Revenues													$ 203,890	$ 174,492	$ 52,194

Segment Information - Schedu114

Segment Information - Schedule of Segment Reporting Information (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total operating revenues	$ 641,525	$ 539,969	$ 547,748	$ 731,248	$ 685,405	$ 505,396	$ 538,604	$ 734,220	$ 627,683	$ 432,475	$ 453,153	$ 608,396	$ 2,460,490	$ 2,463,625	$ 2,121,707
Interest revenue													1,849	2,173	2,602
Interest expense													73,660	71,879	72,069
Depreciation and amortization													289,132	270,111	253,027
Income tax expense													78,468	79,902	78,373
Segment net income	65,180	$ 2,472	$ 8,943	$ 75,446	66,119	$ (4,734)	$ 4,949	$ 71,983	58,746	$ 1,970	$ 9,627	$ 70,783	152,041	138,317	141,126
Segment assets	5,581,126				5,358,685				5,208,297				5,581,126	5,358,685	5,208,297
Capital expenditures													529,531	488,000	396,898
Revenues from unaffiliated customers [Member]
Segment Reporting Information [Line Items]
Revenues													2,362,369	2,359,509	2,029,519
Intersegment sales [Member]
Segment Reporting Information [Line Items]
Revenues													98,121	104,116	92,188
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Total operating revenues													1,321,412	1,454,639	1,382,087
Interest revenue													1,848	1,754	2,596
Interest expense													66,997	64,095	68,299
Depreciation and amortization													233,463	213,455	204,144
Income tax expense													58,584	61,355	63,597
Segment net income													119,423	111,625	116,872
Segment assets	5,001,756				4,822,845				4,652,307				5,001,756	4,822,845	4,652,307
Capital expenditures													457,120	438,289	350,025
Natural Gas Operations [Member] | Revenues from unaffiliated customers [Member]
Segment Reporting Information [Line Items]
Revenues													1,321,412	1,454,639	1,382,087
Natural Gas Operations [Member] | Intersegment sales [Member]
Segment Reporting Information [Line Items]
Revenues													0	0	0
Construction Services [Member]
Segment Reporting Information [Line Items]
Total operating revenues													1,139,078	1,008,986	739,620
Interest revenue													1	419	6
Interest expense													6,663	7,784	3,770
Depreciation and amortization													55,669	56,656	48,883
Income tax expense													19,884	18,547	14,776
Segment net income													32,618	26,692	24,254
Segment assets	$ 579,370				$ 535,840				566,589				579,370	535,840	566,589
Capital expenditures													72,411	49,711	46,873
Construction Services [Member] | Revenues from unaffiliated customers [Member]
Segment Reporting Information [Line Items]
Revenues													1,040,957	904,870	647,432
Construction Services [Member] | Intersegment sales [Member]
Segment Reporting Information [Line Items]
Revenues													$ 98,121	$ 104,116	92,188
Adjustments [Member]
Segment Reporting Information [Line Items]
Segment assets									$ (10,599)						$ (10,599)

Segment Information - Schedu115

Segment Information - Schedule of Segment Reporting Information (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Long-term deferred tax benefit	$ 67,881	$ 48,032	$ 69,368
Natural Gas Operations [Member]
Segment Reporting Information [Line Items]
Deferred income tax liability			4,500
Construction Services [Member]
Segment Reporting Information [Line Items]
Income tax payable netted			3,300
Deferred income tax liability			1,400
Long-term deferred tax benefit			$ 1,400

Quarterly Financial Data - Sche

Quarterly Financial Data - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 641,525	$ 539,969	$ 547,748	$ 731,248	$ 685,405	$ 505,396	$ 538,604	$ 734,220	$ 627,683	$ 432,475	$ 453,153	$ 608,396	$ 2,460,490	$ 2,463,625	$ 2,121,707
Operating income	117,963	15,539	28,116	134,096	117,586	16,143	25,047	129,556	112,373	18,290	26,755	127,065	295,714	288,332	284,483
Net income (loss)	65,694	2,907	9,099	75,355	66,698	(4,210)	5,063	71,879	58,897	1,927	9,627	70,697	153,055	139,430	141,148
Net income (loss) attributable to Southwest Gas Corporation	$ 65,180	$ 2,472	$ 8,943	$ 75,446	$ 66,119	$ (4,734)	$ 4,949	$ 71,983	$ 58,746	$ 1,970	$ 9,627	$ 70,783	$ 152,041	$ 138,317	$ 141,126
Basic earnings (loss) per common share	$ 1.37	$ 0.05	$ 0.19	$ 1.59	$ 1.40	$ (0.10)	$ 0.11	$ 1.54	$ 1.26	$ 0.04	$ 0.21	$ 1.52	$ 3.20	$ 2.94	$ 3.04
Diluted earnings (loss) per common share	$ 1.36	$ 0.05	$ 0.19	$ 1.58	$ 1.38	$ (0.10)	$ 0.10	$ 1.53	$ 1.25	$ 0.04	$ 0.21	$ 1.51	$ 3.18	$ 2.92	$ 3.01

Construction Services Noncon117

Construction Services Noncontrolling Interests - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Oct. 31, 2014	Dec. 31, 2016
Principal Previous Owners [Member] | Link-Line and WS Nicholls [Member]
Noncontrolling Interest [Line Items]
Equity interest percentage	10.00%
Description of equity interest		Associated with the agreement reached in conjunction with the acquisition of the Canadian construction businesses in October 2014, the previous owners of the acquired companies initially retained an approximate 10% equity interest in the Canadian-specific businesses, and special dividend rights which entitled the sellers, as holders, to dividends equal to 3.4% of dividends paid at the level of Centuri, and subject to certain conditions, such interests could become exchangeable for a 3.4% equity interest in Centuri. In consideration of the underlying exchange rights of the original agreement, earnings attribution by Centuri to the previous owners also occurred in an amount equivalent to 3.4% of Centuri earnings since October 2014. During the third quarter of 2015, the sellers formally exercised their exchange rights under the terms of the original agreement. No new rights were conveyed to the noncontrolling parties as a result of the exchange and no new consideration was involved. The previous owners are currently eligible to exit their investment retained by requiring the purchase of a portion of their interest and in incremental amounts annually. The shares subject to the election cumulate (if earlier elections are not made) such that 100% of their interest retained is subject to the election beginning in July 2022.
Equity interest dividend rights, percentage of dividends paid by Centuri	3.40%
Principal Previous Owners [Member] | Link-Line and WS Nicholls [Member] | Construction Services [Member]
Noncontrolling Interest [Line Items]
Equity interest exchangeable percentage in Centuri	3.40%
Percentage of earnings attribution by Centuri to previous owners	3.40%
Principal Previous Owners [Member] | Link-Line and WS Nicholls [Member] | Construction Services [Member] | Maximum [Member]
Noncontrolling Interest [Line Items]
Percentage of retained interest may elect to sell to centuri		100.00%
IntelliChoice Energy, LLC [Member]
Noncontrolling Interest [Line Items]
Subsidiaries holding interest percentage		65.00%

Construction Services Noncon118

Construction Services Noncontrolling Interests - Summary of Redeemable Noncontrolling Interest (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Redeemable Noncontrolling Interest [Line Items]
Balance, December 31, 2015	$ 16,108
Balance, December 31, 2016	22,590	$ 16,108
Redeemable Noncontrolling Interest (Temporary Equity) [Member]
Redeemable Noncontrolling Interest [Line Items]
Balance, December 31, 2015	16,108	20,042
Net Income (loss) attributable to redeemable noncontrolling interest	1,148	939	$ 151
Foreign currency exchange translation adjustment	5	(66)	(22)
Centuri distribution to redeemable noncontrolling interest	(439)	(99)
Adjustment to redemption value	5,768
Balance, December 31, 2016	$ 22,590	$ 16,108	$ 20,042