Long-Term Debt Revolving Credit Facility (Details) - USD ($) $ in Thousands		Jul. 25, 2018	Jun. 30, 2018	Apr. 24, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Credit facility balance			$ 0		$ 0
Line of Credit [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit agreement, maximum loan amount			2,500,000
Line of credit facility, current borrowing base			1,300,000	$ 1,400,000
Line of credit agreement, aggregate lender commitments			1,000,000	$ 1,000,000	925,000
Credit facility balance	[1]		0		0
Letters of credit	[2]		200		200
Available borrowing capacity			999,800		924,800
Unamortized deferred financing costs			$ 2,400		$ 3,100
Subsequent Event [Member] | Line of Credit [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit agreement, aggregate lender commitments		$ 1,000,000
Credit facility balance		0
Letters of credit	[2]	200
Available borrowing capacity		$ 999,800

[1] (1) Unamortized deferred financing costs attributable to the credit facility are presented as a component of other noncurrent assets on the accompanying balance sheets and totaled $2.4 million and $3.1 million as of June 30, 2018, and December 31, 2017, respectively.

[2] (2) Letters of credit outstanding reduce the amount available under the credit facility on a dollar-for-dollar basis.