Investment in ATC (Details-2) - ATC [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule Of Equity Method Investments [Line Items]
Operating revenues	$ 154,225	$ 165,171	$ 318,465	$ 317,528
Operating expenses	(81,698)	(80,326)	(160,763)	(160,277)
Other income (expense), net	1,308	(81)	1,435	(19)
Interest expense, net	(24,882)	(24,172)	(49,090)	(48,655)
Earnings before members' income taxes	$ 48,953	$ 60,592	$ 110,047	$ 108,577