Other Financial Information - Noncash Operating Working Capital (Increase) Decrease) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net (increase) decrease in operating working capital, excluding cash and cash equivalents:
(Increase) decrease in accounts receivable	$ 164,613	$ (232,037)	$ (30,212)
(Increase) decrease in inventories	5,953	10,258	16,794
(Increase) decrease in prepaid expenses	7,178	4,650	(10,011)
Increase (decrease) in accounts payable and accrued liabilities	(208,740)	196,773	8,784
Increase (decrease) in income taxes payable	(1,031)	3,469	(1,458)
Net (increase) decrease in noncash operating working capital	(32,027)	(16,887)	(16,103)	[1]
Supplementary disclosures:
Cash income taxes paid, net of refunds	(44,715)	(6,645)	(7,603)
Interest paid, net of amounts capitalized of $8.0 million in 2020, $1.8 million in 2019 and $0.2 million in 2018	191,561	179,722	158,071
Capitalized interest paid	8,000	1,800	200
Non-cash investing activities:
Asset retirement costs capitalized	14,736	33,874	346,387
(Increase) decrease in capital expenditure accrual	$ 84,645	$ (73,426)	$ 9,817

[1] Reclassified to conform with current presentation