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Long-term debt: (Details) (USD $)
12 Months Ended 0 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jan. 26, 2011
Senior notes
Dec. 31, 2012
Senior notes
Dec. 31, 2011
Senior notes
Dec. 31, 2012
Senior notes
Prior to February 15, 2015
Dec. 31, 2012
Senior notes
12-month period beginning on February 15, 2015
Dec. 31, 2012
Senior notes
12-month period beginning on February 15, 2016
Dec. 31, 2012
Senior notes
12-month period beginning on February 15, 2017 and thereafter
Dec. 31, 2012
Senior notes
Prior to February 15, 2014
Jun. 30, 2007
Convertible senior notes
Dec. 31, 2012
Convertible senior notes
Dec. 31, 2011
Convertible senior notes
Dec. 31, 2010
Convertible senior notes
Dec. 31, 2008
Convertible senior notes
Dec. 31, 2012
Convertible senior notes
On or after June 20, 2014
Dec. 31, 2012
Convertible senior notes
June 15, 2014, 2017 and 2022
Long-term debt                                    
Interest rate (as a percent)       8.375%               1.00% 1.00% 1.00%        
Aggregate principal amount of debt issued       $ 175,000,000               $ 200,000,000            
Proceeds from issuance of long-term debt, net of issuance costs       170,500,000               195,100,000            
Debt issuance costs       4,500,000                            
Interest expense related to its senior notes         15,200,000 14,000,000                        
Security of senior notes and guarantees, expressed as a percentage of equity interests of first-tier foreign subsidiaries held by Company and its guarantors         65.00%                          
Percentage of outstanding principal amount used in calculation of make-whole premium             1.00%                      
Discount rate used to compute make-whole premium, description of variable interest rate         Treasury Rate                          
Discount rate used to compute make-whole premium, basis points added to reference rate (as a percent)         0.50%                          
Redemption price of debt instrument (as a percent)             100.00% 104.118% 102.094% 100.00% 108.375%           100.00%  
Maximum percentage of principal amount of debt instrument which the entity may redeem with proceeds from certain equity offerings                     35.00%              
Percentage of principal amount at which notes will be required to be repurchased in the event of a change of control         101.00%                          
Minimum amount of proceeds from certain asset sales at which company must offer to purchase notes         20,000,000                          
Percentage of principal amount at which notes will be required to be repurchased upon meeting minimum proceeds from certain asset sales         100.00%                          
Minimum percentage of principal amount of debt instrument at which holder may declare unpaid principal and accrued interest to be due and payable         25.00%                          
Percentage of original issuance discount                       2.25%            
Aggregate face value of debt purchased                               108,000,000    
Purchase of convertible notes in cash                               48,600,000    
Initial conversion price of notes (in dollars per share)                         $ 49.18          
Conversion ratio, number of shares per $1,000 principal amount                         0.0203355          
Number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be redeemable                         20 days          
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order to make the notes redeemable                         30 days          
Percentage of the closing sales price of the entity's common stock that the conversion price must exceed in order to make the notes convertible                         130.00%          
Conversion ratio minimum, number of shares per $1,000 principal amount                         0.0355872          
Percentage of principal amount that the holders of the Convertible Notes may require the Company to repurchase                                   100.00%
Special interest rate required to be paid for first 90 days after failure to maintain shelf registration statement with SEC (as a percent)                         0.25%          
Initial period to charge special interest after failure to maintain shelf registration statement with SEC                         90 days          
Special interest rate required to be paid after the first 90 day period after failure to maintain shelf registration statement with SEC (as a percent)                         0.50%          
Additional interest rate in the event of default (as a percent)                         0.25%          
Debt and equity components for the Convertible Notes                                    
Principal amount                         91,978,000 91,978,000        
Unamortized discount (9,494,000) (15,366,000)                     (9,494,000) (15,366,000)        
Net carrying amount 82,484,000 76,612,000                     82,484,000 76,612,000        
Additional paid-in capital                         74,933,000 74,933,000        
Amount of interest expense recognized and effective interest rate                                    
Remaining recognition period of unamortized discount                         1 year 6 months          
Contractual coupon interest                         920,000 920,000 920,000      
Amortization of discount and costs 6,031,000 5,609,000 4,950,000                   5,886,000 5,405,000 4,964,000      
Interest expense                         6,806,000 6,325,000 5,884,000      
Effective interest rate (as a percent)                         8.70% 8.70% 8.70%      
Long-term debt maturities                                    
Thereafter                         266,978,000          
Total                         $ 266,978,000