Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 182,212	$ 165,506	$ 359,927	$ 330,935
Operating expenses	(93,573)	(91,474)	(183,960)	(176,367)
Other income, net	287	1,520	547	1,631
Interest expense, net	(28,838)	(26,895)	(57,956)	(54,657)
Earnings before members' income taxes	60,088	48,657	118,558	101,542
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,600	$ 7,200	15,200	$ 14,400
Due from related parties	$ 600		$ 600		$ 100

The amount of interest reported by an equity method investment of the entity.

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The amount of net income or net loss before taxes reported by an equity method investment of the entity.

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The amount of other income or expenses reported by an equity method investment of the entity.

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For an unclassified balance sheet, amounts due from related parties including affiliates, employees, joint ventures, officers and stockholders, immediate families thereof, and pension funds.

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The amount of the cost of sales reported by an equity method investment of the entity.

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The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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