Debt (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Debt Instrument [Line Items]
Equity component		$ 4,938,533	$ 4,883,670
Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Principal		1,437,483	1,437,483
Less: note discount and issuance costs		(41,436)	(51,159)
Net carrying amount		1,396,047	1,386,324	$ 1,440,000
Equity component	[1]	$ 162,869	$ 162,869

[1] Recorded in the condensed consolidated balance sheet within additional paid-in-capital.