Commitments And Contingencies - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) derivative_instrument	Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items]
Notes payable, fair value disclosure	$ 601,000		$ 601,000
Number of instruments held | derivative_instrument						2
Interest rate swap principal	200,000		200,000			$ 200,000	$ 200,000
Proceeds from Sale of Debt Securities, Available-for-sale			12,840	$ 0	$ 70,000
Debt Securities, Available-for-sale	10,000		10,000		$ 10,000		10,000
Fair Value, Inputs, Level 2 [Member]
Loss Contingencies [Line Items]
Investments, fair value disclosure	10,000		10,000				10,000
Angola, Kwanza [Member]
Loss Contingencies [Line Items]
Foreign Currency Transaction Gain (Loss), before Tax	(200)	$ 600	(2,200)	$ (600)
Cash and cash equivalents	$ 11,000		$ 11,000				$ 6,200
Senior Notes due 2024 [Member]
Loss Contingencies [Line Items]
Fixed interest rate						4.65%
Derivative Liability, Notional Amount						$ 100,000
Senior Notes due 2024 [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Loss Contingencies [Line Items]
Derivative, Variable Interest Rate						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%