Schedule I - Condensed Financial Information Of Registrant (Unconsolidated Statements Of Cash Flows) (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 01, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income		$ 108,793	$ 131,060	$ 59,524
Amortization of debt issuance costs and discounts		(4,202)	(4,147)	(5,067)
Income taxes receivable or payable		(12,592)	(1,603)	0
Accounts payable and accrued expenses		(5,345)	13,668	(716)
Other - net		(1,195)	3,559	6,655
Net cash provided by operating activities		285,260	324,591	252,425
Net cash used in investing activities		(236,432)	(608,688)	(695,117)
Long-term borrowings, net of discount		404,633	387,662	753,350
Retirement of long-term debt		(408,152)	(40,000)	(552,179)
Dividends on common stock		(105,144)	(122,959)	(69,487)
Issuance of common stock		0	134,276	214,366
Other - net		228	153	368
Net cash (used in) provided by financing activities		(53,100)	297,529	437,280
Net change in cash and cash equivalents		(4,272)	13,432	(5,412)
Cash and cash equivalents at beginning of period		34,953	21,521	26,933
Cash and cash equivalents at end of period		30,681	34,953	21,521
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income		105,580	127,847	56,311
Equity in earnings of subsidiaries		(133,725)	(153,232)	(98,708)
Cash dividends received from subsidiary companies		132,516	136,466	106,997
Amortization of debt issuance costs and discounts		(2,003)	(1,947)	(2,459)
Deferred income taxes - net		78	22,601	(2,190)
Charges related to early extinguishment of debt		8,875	0	21,956
Income taxes receivable or payable		5,377	(5,425)	2,465
Accounts payable and accrued expenses		203	(800)	(391)
Other - net		370	145	639
Net cash provided by operating activities		121,277	129,549	89,538
Investment in subsidiaries		0	(212,997)	(214,366)
Net cash used in investing activities		0	(212,997)	(214,366)
Long-term borrowings, net of discount		404,633	0	404,712
Retirement of long-term debt		(408,152)	0	(420,329)
Dividends on common stock		(105,144)	(122,959)	(69,487)
Issuance of common stock		0	134,276	214,366
Equity contributions from AES	$ 64,800	0	78,738	0
Other - net		3,601	11	5,445
Net cash (used in) provided by financing activities		(112,264)	90,044	123,817
Net change in cash and cash equivalents		9,013	6,596	(1,011)
Cash and cash equivalents at beginning of period		7,370	774	1,785
Cash and cash equivalents at end of period		$ 16,383	$ 7,370	$ 774