Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash paid for interest, expensed	$ 50,076	$ 98,261	$ 130,843
Cash paid for interest, capitalized	37,079	30,762	25,982
Cash paid for interest, treated as a reduction of debt	79,606	50,349	25,835
Cash paid for income taxes	492	450	375
Cash received from income tax refunds	(8,280)	(13,323)	(2,455)
Noncash investing and financing activities
Increase in asset retirement obligations	4,499	9,565	11,621
Increase (decrease) in liabilities for capital expenditures	14,600	3,930	(13,593)
Conversion of convertible senior notes into common stock	162,004	0	0
Other Significant Noncash Transactions [Line Items]
Retirement of treasury stock		0
Treasury Stock
Other Significant Noncash Transactions [Line Items]
Retirement of treasury stock	$ 0	$ 46,562	$ 0