CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME/(LOSS) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income/(loss)	$ 53,645,000	$ (6,307,000)	$ 3,211,000	$ 5,429,000	$ (275,475,000)	$ (98,739,000)	$ 29,861,000	$ 50,739,000	$ 55,978,000	$ (293,614,000)	$ 283,960,000
Other Comprehensive Income/(Loss), net of tax:
Net change in unrealized gains on cash flow hedges									907,000	10,311,000	6,927,000
Defined benefit pension and other postretirement benefit plans:
Net change in unrecognized prior service costs									(157,000)	(60,000)	(211,000)
Net change in unrecognized actuarial losses									948,000	(3,295,000)	3,203,000
Other comprehensive income									1,698,000	6,956,000	9,919,000
Comprehensive income/(loss)									$ 57,676,000	$ (286,658,000)	$ 293,879,000