Net Income (Loss) Per Share (Narrative) (Details) - Convertible Senior Notes [Member] - USD ($)	Mar. 31, 2019	Dec. 31, 2018	Aug. 31, 2018	Aug. 24, 2018	Jun. 30, 2014
2019 Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Convertible senior notes, issued	$ 65,985,000	$ 65,987,000			$ 330,000,000
Convertible senior notes, interest rate					2.25%
2028 Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Convertible senior notes, issued	$ 880,000,000	$ 880,000,000	$ 880,000,000
Convertible senior notes, interest rate			2.00%	2.00%