Commitments And Contingencies - Narrative (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) derivative_instrument	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) derivative_instrument	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Notes payable, fair value disclosure	$ 761,000,000		$ 761,000,000
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	6,900,000		6,900,000
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]
Foreign Currency Transaction Gain (Loss), before Tax	2,000,000.0	$ 4,500,000	2,600,000	$ 17,000,000
Cash and cash equivalents	8,400,000		8,400,000		$ 9,300,000
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 Liability, 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%