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Long-Term Debt - Revolving Credit and Convertible Notes (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Nov. 19, 2013
USD ($)
shares
Dec. 31, 2017
USD ($)
item
$ / shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Long-Term Debt        
Repayment of outstanding borrowings   $ 976,161 $ 311,362 $ 499,551
Revolving Credit Facility        
Long-Term Debt        
Maximum borrowing capacity   400,000    
Available borrowing capacity under revolving credit facility   277,500    
Sublimit for swingline loans   50,000    
Sublimit for letters of credit   30,000    
Amount outstanding   $ 122,500    
Weighted average interest rate, as a percent   3.20% 4.00%  
Accordion Provision Under Credit Facility [Member]        
Long-Term Debt        
Maximum borrowing capacity   $ 500,000    
Convertible Senior Notes 1.00 percent due 2020        
Long-Term Debt        
Face amount of debt issuance   $ 287,500    
Interest rate, as a percentage   1.00%    
Net proceeds after fees and shares repurchase $ 254,200      
Shares repurchased | shares 665,994      
Fair value of debt component $ 215,800      
Debt instrument, fair value of embedded option $ 71,700 $ 71,700    
Principal balance   $ 287,500 $ 287,500  
Effective interest rate percentage 5.26%      
Conversion price (in dollars per share) | $ / shares   $ 52.35    
Conversion ratio per $1 principal amount   19.1022    
Number of shares to be received upon conversion of debt | item   5,500,000    
Threshold percentage of share price that triggers conversion   135.00%    
Debt conversion, threshold trading days | item   20    
Debt conversion, threshold consecutive trading days | item   30    
Debt Instrument, Convertible, Consecutive Trading Days Available for Conversion   10 days    
Debt Instrument, Convertible, Threshold Consecutive Trading Days   5 days    
Debt Instrument, Convertible, Threshold Percentage of Conversion   98.00%    
Percentage of the total voting power of all classes of common equity as defined as a fundamental change per the terms of the debt agreement   50.00%    
Amount convertible at end of period   $ 0    
Change in control conversion price, as percentage of par plus interest   100.00%    
Minimum | Credit Agreement        
Long-Term Debt        
Fixed Charge Coverage Ratio, debt covenant   1.50    
Minimum | Credit Agreement | Alternative Base Rate Loans And Canadian Prime Rate Loans [Member]        
Long-Term Debt        
Margin (as a percent)   0.00%    
Minimum | Credit Agreement | Adjusted LIBOR Loans, Canadian Dealer Offered Rate Loans And Bank Bill Swap Reference Rate Loans [Member]        
Long-Term Debt        
Margin (as a percent)   1.00%    
Minimum | Credit Agreement | Johannesburg Interbank Agreed Rate Loans [Member]        
Long-Term Debt        
Margin (as a percent)   1.25%    
Maximum | Credit Agreement        
Long-Term Debt        
Senior secured net leverage ratio, debt covenant   2.25    
Total net leverage ratio, debt covenant   4.00    
Total net leverage ratio related to restricted payments   3.00    
Maximum | Credit Agreement | Alternative Base Rate Loans And Canadian Prime Rate Loans [Member]        
Long-Term Debt        
Margin (as a percent)   1.25%    
Maximum | Credit Agreement | Adjusted LIBOR Loans, Canadian Dealer Offered Rate Loans And Bank Bill Swap Reference Rate Loans [Member]        
Long-Term Debt        
Margin (as a percent)   2.25%    
Maximum | Credit Agreement | Johannesburg Interbank Agreed Rate Loans [Member]        
Long-Term Debt        
Margin (as a percent)   2.50%