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Fair Value Measurements - Schedule of financial instruments that are not carried at fair value (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Assets:    
Carrying Amount, Short-term investments $ 0 [1] $ 21,923
Carrying Amount, Long-term Investments 0 [1] 69,699
Assets:    
Fair Value, Short-term investments 0 [1] 21,858
Fair Value, Long-term investments 0 [1] 69,090
2023 Notes [Member]    
Liabilities:    
Notes Payable 750,000 750,000
Liabilities:    
Notes Payable Fair Value Disclosure 759,375 736,725
2027 Notes [Member]    
Liabilities:    
Notes Payable 1,000,000 500,000
Liabilities:    
Notes Payable Fair Value Disclosure $ 1,010,000 $ 482,500
[1] Investments were measured at fair value as of March 31, 2019. See table above.