Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,314,779)
Amounts arising during the period	19,192
Amounts reclassified from AOCL to earnings	(2,436)
Net change	16,756
Balance at end of period	(1,246,050)
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	19,192
Amounts reclassified from AOCL to earnings	(262)
Net change	18,930
Balance at end of period	20,490
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	129,431
Balance at end of period	146,187
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	(4,994)
Net change	(4,994)
Balance at end of period	123,855
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	(78)
Net change	(78)
Balance at end of period	326
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	(374)
Net change	(374)
Balance at end of period	6,895
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	3,272
Net change	3,272
Balance at end of period	$ (5,379)