Components of Accumulated Other Comprehensive Loss (Changes in Accumulated Other Comprehensive Loss) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	$ (1,314,779)
Amounts arising during the period	8,594
Amounts reclassified from AOCL to earnings	(1,247)
Net change	7,347
Balance at end of period	(1,278,386)
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	8,594
Amounts reclassified from AOCL to earnings	(81)
Net change	8,513
Balance at end of period	10,073
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	129,431
Balance at end of period	136,778
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	(2,536)
Net change	(2,536)
Balance at end of period	126,313
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	(39)
Net change	(39)
Balance at end of period	365
Other Postretirement Benefit Plans [Member] | Actuarial Loss [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance at beginning of period	7,269
Amounts reclassified from AOCL to earnings	(227)
Net change	(227)
Balance at end of period	7,042
Other Postretirement Benefit Plans [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	1,636
Net change	1,636
Balance at end of period	$ (7,015)