CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Parent [Line Items]
Net income (loss)	$ 12,836	$ 2,335	$ 175,662	$ 208,074
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	(266)	(29)	400	(400)
Other comprehensive income (loss)	(266)	(29)	400	(400)
Comprehensive income (loss)	12,570	2,306	176,062	207,674
Subsidiaries [Member]
Parent [Line Items]
Net income (loss)	29,100	18,977	222,846	256,382
Other comprehensive income (loss):
Net unrealized gain (loss) from pension and postretirement plans, net of tax	1,744	2,642	7,083	7,322
Amortization of treasury interest rate swaps to earnings, net of tax	79	79	237	237
Other comprehensive income (loss)	1,823	2,721	7,320	7,559
Comprehensive income (loss)	$ 30,923	$ 21,698	$ 230,166	$ 263,941