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Long-Term Debt, Convertible Note (Details)
 12 Months Ended
Jun. 16, 2016
USD ($)
Dec. 31, 2017
USD ($)
d
$ / shares
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Convertible Debt [Abstract]        
Equity component of convertible notes allocated to additional paid-in capital     $ 6,825,000  
Net Carrying Amount of Liability Component [Abstract]        
Principal [1]   $ 401,768,000    
Unamortized debt discount (conversion option)   (9,100,000)    
Unamortized debt discount (issuance costs)   (4,000,000)    
Debt issuance costs   0 6,596,000 $ 0
Interest expense   22,200,000 15,600,000 $ 7,900,000
Convertible Note [Member]        
Convertible Debt [Abstract]        
Original principal amount $ 210,000,000 $ 210,000,000    
Interest rate   4.75%    
Maturity date   Jun. 15, 2020    
Redemption price, percentage of principal amount redeemed   100.00%    
Threshold percentage of stock price trigger   180.00%    
Threshold trading days | d   20    
Threshold final trading days | d   3    
Threshold consecutive trading days | d   30    
Principal amount increments that can be repurchased upon a change in control or termination of trading of common stock   $ 1,000    
Principal amount increments that can be converted   $ 1,000    
Holding period following issue date before notes can be converted   6 months    
Initial conversion rate (in shares) | shares   21.5054    
Initial conversion price (in dollars per share) | $ / shares   $ 46.50    
Proceeds from issuance of notes 210,000,000      
Liability component of convertible notes 199,100,000      
Equity component of convertible notes allocated to additional paid-in capital $ 10,900,000      
Effective interest rate   7.10%    
Net Carrying Amount of Liability Component [Abstract]        
Principal   $ 210,000,000 $ 210,000,000  
Unamortized debt discount (conversion option)   (7,078,000)    
Total long-term debt, net   202,922,000    
Unamortized debt discount (issuance costs)   (4,050,000)    
Net carrying amount   198,872,000    
Debt issuance costs   6,596,000    
Additions to deferred financing cost   6,300,000    
Adjustments to additional paid-in capital for debt issuance costs   300,000    
Interest expense   14,000,000    
Contractual interest   10,000,000    
Amortization of debt issuance costs and debt discount   $ 4,000,000    
[1] The carrying value of the debt reflects the amounts stated in the above table less a debt discount of $13.1 million, which is not reflected in this table.