Long-Term Debt Revolving Credit Facility (Details) - USD ($) $ in Thousands		Apr. 25, 2018	Mar. 31, 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			925,000
Credit facility balance	[1]		0	0
Letters of credit	[2]		200	200
Available borrowing capacity			924,800	924,800
Line of credit agreement, aggregate lender commitments			925,000	925,000
Unamortized deferred financing costs			$ 2,700	$ 3,100
Subsequent Event [Member] | Line of Credit [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, current borrowing base		$ 1,400,000
Credit facility balance		0
Letters of credit	[2]	200
Available borrowing capacity		999,800
Line of credit agreement, aggregate lender commitments		$ 1,000,000

[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 $2.7 million and $3.1 million as of March 31, 2018, and December 31, 2017, respectively.

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