Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,314,779)
Amounts arising during the period	26,576
Amounts reclassified from AOCL to earnings	(3,929)
Net change	22,647
Balance at end of period	(1,235,729)
Gains and losses on cash flow hedges [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	1,560
Amounts arising during the period	26,576
Amounts reclassified from AOCL to earnings	(668)
Net change	25,908
Balance at end of period	27,468
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	129,431
Balance at end of period	152,078
Pension Plans [Member] | Actuarial Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	128,849
Amounts reclassified from AOCL to earnings	(7,490)
Net change	(7,490)
Balance at end of period	121,359
Pension Plans [Member] | Prior Service Cost [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	404
Amounts reclassified from AOCL to earnings	(117)
Net change	(117)
Balance at end of period	287
OPEB [Member] | Actuarial Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	7,269
Amounts reclassified from AOCL to earnings	(563)
Net change	(563)
Balance at end of period	6,706
OPEB [Member] | Prior Service Cost [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	(8,651)
Amounts reclassified from AOCL to earnings	4,909
Net change	4,909
Balance at end of period	$ (3,742)