CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Parent [Line Items]
Net income (loss)	$ 235,622	$ 175,194	$ 312,899
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	(47,690)	9,430	(6,446)
Other Comprehensive Income Reclassification Of Net Unrealized Gain Loss On Interest Rate Swaps During Period Net Of Tax	5,230	0	0
Other comprehensive income (loss)	(52,920)	9,430	(6,446)
Comprehensive income (loss)	182,702	184,624	306,453
Subsidiaries [Member]
Parent [Line Items]
Net income (loss)	317,162	320,054	380,581
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	(37,030)	18,288	3,722
Reclassification of net unrealized loss on energy derivative instruments settled during the period, net of tax	385	317	317
Amortization of treasury interest rate swaps to earnings, net of tax	(27,333)	0	0
Other comprehensive income (loss)	(63,978)	18,605	4,039
Comprehensive income (loss)	$ 253,184	$ 338,659	$ 384,620