Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset	$ 61,191	$ 76,865
Liability	150,381	210,512
Cash and cash equivalents	8,507	10,484	$ 22,143	$ 82,574
Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	400	600
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(50,546)	(74,634)
Interest rate swap derivatives	8,750	1,548
Total	17,915	9,719
Interest Rate Derivative Liabilities, at Fair Value	34,730	85,498
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(95,304)	(89,160)
Total	55,077	121,352
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(46,099)	(73,954)
Derivative Asset	1,895	683
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(55,957)	(88,480)
Derivative Liability	914	8,713
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(69)	(678)
Derivative Asset	0	0
Liability	5,885
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(69)	(678)
Derivative Liability	5,885	5,039
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(25)
Derivative Asset	0
Liability	13,449
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(25)	0
Derivative Liability	13,449	21,961
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability	76
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	0	0
Derivative Liability	76	124
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(4,348)	0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(39,248)	0
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset	(5)	(2)
Derivative Asset	0	0
Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset	(5)	(2)
Derivative Liability	23	17
Fair Value, Measurements, Recurring [Member] | Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	1,789	1,727
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	5,481	5,761
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	7,270	7,488
Total	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	1,789	1,727
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	5,481	5,761
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap derivatives	13,098	1,548
Total	61,097	76,187
Interest Rate Derivative Liabilities, at Fair Value	73,978	85,498
Total	130,877	182,710
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Energy commodity derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset	47,994	74,637
Liability	56,871	97,193
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign Exchange Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset	5	2
Liability	28	19
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	94	678
Total	19,504	27,802
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset	69	678
Liability	5,954	5,717
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset	25
Liability	13,474	21,961
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Commodity Option [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability	$ 76	$ 124