Investment in ATC and ATC Holdco (Details-2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
ATC [Member]
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 176,610	$ 154,225	$ 351,279	$ 318,465
Operating expenses	(82,665)	(81,698)	(165,053)	(160,763)
Other income, net	612	1,308	819	1,435
Interest expense, net	(26,299)	(24,882)	(52,915)	(49,090)
Earnings before members' income taxes	68,258	48,953	134,130	110,047
ATC [Member]
Related Party Transaction [Line Items]
Related party expenses	7,300	$ 7,400	14,600	$ 14,700
Due from related parties	$ 100		$ 100		$ 100