Commitments And Contingencies - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss Contingencies [Line Items]
Notes Payable, Fair Value Disclosure	$ 495,000
Letters of Credit Outstanding, Amount	54,000	$ 58,000
Other Nonoperating Income (Expense)	(6,244)	(15,336)	$ (387)
Cash and Cash Equivalents, at Carrying Value	450,193	385,235	430,714	$ 91,430
Investments and Other Noncurrent Assets	59,130
Derivative, Amount of Hedged Item	200,000
Fair Value Hedge Liabilities	(1,557)	1,909
Other Income [Member]
Loss Contingencies [Line Items]
Other Nonoperating Income (Expense)	(7,300)	(17,400)	$ 400
Other Noncurrent Liabilities [Member]
Loss Contingencies [Line Items]
Fair Value Hedge Liabilities	(2,171)
Angola, Kwanza
Loss Contingencies [Line Items]
Cash and Cash Equivalents, at Carrying Value	32,000
Angola Member
Loss Contingencies [Line Items]
Investments and Other Noncurrent Assets	59,130	$ 0
Other Noncurrent Assets [Member]
Loss Contingencies [Line Items]
Fair Value Hedge Assets	600
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Investments, Fair Value Disclosure	$ 57,500