Schedule I - Condensed Financial Information Of Registrant (Unconsolidated Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	$ 30,681	$ 34,953	$ 21,521	$ 26,933
Prepayments and other current assets	34,094	33,504
Total current assets	353,709	347,840
Other long-term assets	6,185	5,916
Other assets - net	22,723	18,404
TOTAL	4,740,561	4,702,281
Paid in capital	597,467	596,810
Accumulated deficit	(25,191)	(25,627)
Total common shareholder's deficit	572,276	571,183
Long-term debt	2,477,538	2,474,840
Total capitalization	3,109,598	3,105,807
Accrued interest	34,332	32,541
Total current liabilities	397,930	321,337
Total deferred credits and other long-term liabilities	1,233,033	1,275,137
TOTAL	4,740,561	4,702,281
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Cash and cash equivalents	16,383	7,370	$ 774	$ 1,785
Prepayments and other current assets	9	10,076
Total current assets	16,392	17,446
Investment in subsidiaries	1,369,100	1,360,235
Other long-term assets	3,585	3,247
Deferred tax asset - long term	22	101
Other assets - net	1,372,707	1,363,583
TOTAL	1,389,099	1,381,029
Paid in capital	597,467	596,810
Accumulated deficit	(25,191)	(25,627)
Total common shareholder's deficit	572,276	571,183
Long-term debt	801,603	799,709
Total capitalization	1,373,879	1,370,892
Accounts payable and accrued expenses	1,102	359
Accrued Income Taxes, Current	2,304	0
Accrued interest	11,813	9,776
Total current liabilities	15,219	10,135
Total deferred credits and other long-term liabilities	1	2
TOTAL	$ 1,389,099	$ 1,381,029