CONSOLIDATED STATEMENT OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income (loss)		$ 557,043	$ 100,393	$ (417,124)
Postretirement benefits:
Net gain (loss)	[1]	2,461	(2,634)	1,794
Prior service credit (cost)	[2]	0	5,449	(514)
Amortization of prior service cost	[3]	(547)	(1,723)	70
Cumulative effect of adoption of ASU 2018-02 reclassified to retained earnings		446	0	0
Total other comprehensive income		2,360	1,092	1,350
Comprehensive income (loss)		$ 559,403	$ 101,485	$ (415,774)

[1] Net of income taxes of $(704), $1,544 and $(1,052) for the year ended December 31, 2018, 2017 and 2016, respectively.

[2] Net of income taxes of $0, $(3,194) and $301 for the year ended December 31, 2018, 2017 and 2016, respectively.

[3] Net of income taxes of $162, $1,010 and $(41) for the year ended December 31, 2018, 2017 and 2016, respectively.