Commitments And Contingencies - Narrative (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019 USD ($) derivative_instrument	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) derivative_instrument	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Notes payable, fair value disclosure	$ 792,000,000		$ 792,000,000
Number of instruments held | derivative_instrument	2		2
Interest rate swap principal	$ 200,000,000		$ 200,000,000		$ 200,000,000
Fair value of interest rate swaps	(2,911,000)		(2,911,000)
Other income (expense), net	(7,000)	$ 3,556,000	(726,000)	$ 12,030,000
Proceeds from sale of debt securities					70,000,000
Debt Securities, Available-for-sale	10,000,000		10,000,000		10,000,000
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Investments, fair value disclosure	10,000,000		10,000,000		10,000,000
Angola, Kwanza [Member]
Loss Contingencies [Line Items]
Cash and cash equivalents	7,100,000		7,100,000		$ 9,300,000
Other Income [Member]
Loss Contingencies [Line Items]
Other income (expense), net	(600,000)	$ (4,800,000)	(600,000)	$ (12,000,000)
Senior Notes due 2024 [Member]
Loss Contingencies [Line Items]
Interest rate swap principal	$ 200,000,000		$ 200,000,000
Fixed interest rate	4.65%		4.65%
Derivative, notional amount	$ 100,000,000		$ 100,000,000
Senior Notes due 2024 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Derivative, variable interest rate	2.426%		2.426%
Senior Notes due 2024 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Derivative, variable interest rate	2.823%		2.823%