Schedule I - Condensed Financial Information Of Registrant (Unconsolidated Statements Of Common Shareholders' Deficit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	$ 571,183	$ 352,933	$ 151,271
Net income applicable to common stock	105,580	127,847	56,311
Distributions to AES	(105,144)	(122,959)	(69,487)
Proceeds from Contributed Capital	0	78,738	0
Issuance of common stock		134,276	214,366
Other	657	348	472
Ending Balance	572,276	571,183	352,933
Parent Company [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	571,183	352,933	151,271
Net income applicable to common stock	105,580	127,847	56,311
Distributions to AES	105,144	122,959	69,487
Issuance of common stock		134,276
Contributions from AES		78,738	214,366
Other	657	348	472
Ending Balance	572,276	571,183	352,933
Paid In Capital [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	596,810	383,448	168,610
Proceeds from Contributed Capital		78,738
Issuance of common stock		134,276	214,366
Other	657	348	472
Ending Balance	597,467	596,810	383,448
Paid In Capital [Member] | Parent Company [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	596,810	383,448	168,610
Issuance of common stock		134,276
Contributions from AES		78,738	214,366
Other	657	348	472
Ending Balance	597,467	596,810	383,448
Accumulated Deficit [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(25,627)	(30,515)	(17,339)
Net income applicable to common stock	105,580	127,847	56,311
Distributions to AES	(105,144)	(122,959)	(69,487)
Ending Balance	(25,191)	(25,627)	(30,515)
Accumulated Deficit [Member] | Parent Company [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance	(25,627)	(30,515)	(17,339)
Net income applicable to common stock	105,580	127,847	56,311
Distributions to AES	105,144	122,959	69,487
Ending Balance	$ (25,191)	$ (25,627)	$ (30,515)