Utility Plant and Leases - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Annual utility depreciation and amortization expense percentage	2.70%	3.00%	3.00%
Amortization of intangibles	$ 7.6	$ 4.1	$ 3.2
Gas, Transmission and Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Annual utility depreciation and amortization expense percentage	3.00%	3.00%	3.00%
Gas plant
Public Utility, Property, Plant and Equipment [Line Items]
Amortization of intangibles	$ 13.6	$ 14.3	$ 14.8

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16225-109274

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Balance Type:	debit
Period Type:	duration