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Financial Commitments (Narrative) (Details)
12 Months Ended
Apr. 19, 2017
USD ($)
Dec. 31, 2017
USD ($)
d / item
$ / shares
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2023
USD ($)
Dec. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
Dec. 31, 2018
USD ($)
Debt Instrument [Line Items]                
Issuance of convertible notes     $ 200,000,000          
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding   $ 736,276,000 759,519,000          
Operating leases, rent expense   $ 27,400,000 28,200,000 $ 17,400,000        
Letters Of Credit [Member]                
Debt Instrument [Line Items]                
Face amount $ 150,000,000              
Senior Notes [Member] | 2017 Senior Notes [Member]                
Debt Instrument [Line Items]                
Imputed interest rate (as a percent)   7.13%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding   $ 492,734,000            
Senior Notes [Member] | 2010 Senior Notes [Member]                
Debt Instrument [Line Items]                
Face amount $ 300,000,000              
Interest rate (as a percent) 7.625%              
Maturity date Nov. 01, 2018              
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding     298,120,000          
Senior Notes [Member] | Private Placement [Member] | 2017 Senior Notes [Member]                
Debt Instrument [Line Items]                
Face amount   $ 500,000,000            
Interest rate (as a percent)   6.875%            
Redemption price, change of control triggering event (as a percent)   101.00%            
Senior Notes [Member] | Circumstance One [Member] | Private Placement [Member] | 2017 Senior Notes [Member]                
Debt Instrument [Line Items]                
Redemption price   100.00%            
Senior Notes [Member] | Circumstance Two [Member] | Private Placement [Member] | 2017 Senior Notes [Member]                
Debt Instrument [Line Items]                
Redemption price   106.875%            
Percentage of notes that can be redeemed   40.00%            
Convertible Notes [Member]                
Debt Instrument [Line Items]                
Face amount   $ 200,000,000            
Interest rate (as a percent)   2.875%            
Maturity date   Jun. 15, 2021            
Conversion price, shares | shares   33.0579            
Conversion price | $ / shares   $ 30.25            
Imputed interest rate (as a percent)   9.39%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding   $ 161,635,000 152,668,000          
Convertible Notes [Member] | Equity Component [Member]                
Debt Instrument [Line Items]                
Conversion feature   46,800,000 46,800,000          
Unamortized debt issuance costs   1,543,000 1,543,000          
Convertible Notes [Member] | Liability Component [Member]                
Debt Instrument [Line Items]                
Face amount   200,000,000 200,000,000          
Issuance of convertible notes   153,200,000            
Unamortized debt issuance costs   5,051,000 5,051,000          
Reclassified amortization of deferred debt issuance costs   $ 13,486,000 4,519,000          
Convertible Notes [Member] | Circumstance One [Member]                
Debt Instrument [Line Items]                
Business day period after consecutive trading day period   5 days            
Consecutive trading day period | d / item   10            
Conversion price, principal amount   $ 1,000            
Percentage of conversion price   98.00%            
Convertible Notes [Member] | Circumstance Two [Member]                
Debt Instrument [Line Items]                
Consecutive trading day period | d / item   30            
Percentage of conversion price   130.00%            
Business day period within consecutive trading day period | d / item   20            
Conversion stock price trigger | $ / shares   $ 39.32            
Term Loan                
Debt Instrument [Line Items]                
Face amount $ 250,000,000              
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding     54,650,000          
Equipment Financing Loans [Member]                
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding   $ 61,100,000 84,900,000          
Term of debt   10 years            
Equipment Financing Loans [Member] | Forecast [Member]                
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Balloon payments           $ 6,300,000 $ 12,400,000  
Mortgages [Member]                
Debt Instrument [Line Items]                
Interest rate (as a percent)   2.50%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding   $ 15,700,000 16,700,000          
Term of debt   7 years            
Mortgages [Member] | Forecast [Member]                
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Balloon payments         $ 7,000,000   $ 2,900,000 $ 2,600,000
Minimum [Member] | Equipment Financing Loans [Member]                
Debt Instrument [Line Items]                
Interest rate (as a percent)   1.90%            
Maximum [Member] | Equipment Financing Loans [Member]                
Debt Instrument [Line Items]                
Interest rate (as a percent)   5.93%            
Credit Facility [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Maximum borrowing capacity   $ 350,000,000            
Available borrowing capacity   $ 350,000,000            
Line of Credit Facility, Expiration Date   Apr. 20, 2022            
Applicable margin and letter of credit fees will be increased percentage per annum.   2.00%            
Weighted-average annual interest rate on borrowings   3.89%            
Fixed charge coverage ratio, Required   1.25            
Unamortized debt issuance costs   $ 6,200,000            
Revolver optional increase removed by amendment   150,000,000            
Credit Facility [Member] | 2014 Credit Facility [Member]                
Debt Instrument [Line Items]                
Maximum borrowing capacity $ 300,000,000              
Line of Credit Facility, Expiration Date May 01, 2018              
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Loan outstanding     $ 147,990,000          
Credit Facility [Member] | Letters Of Credit [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Maximum borrowing capacity   150,000,000            
Credit Facility [Member] | Swingline Loans [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Face amount   $ 10,000,000            
Credit Facility [Member] | Minimum [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Consolidated leverage ratio   3.25            
Credit Facility [Member] | Maximum [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Consolidated leverage ratio   4.00            
LIBOR [Member] | Mortgages [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   3.00%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   3.00%            
LIBOR [Member] | Credit Facility [Member] | Minimum [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   1.50%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   1.50%            
LIBOR [Member] | Credit Facility [Member] | Minimum [Member] | Term Loan | 2014 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent)) 1.25%              
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent) 1.25%              
LIBOR [Member] | Credit Facility [Member] | Maximum [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   3.00%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   3.00%            
LIBOR [Member] | Credit Facility [Member] | Maximum [Member] | Term Loan | 2014 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent)) 3.00%              
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent) 3.00%              
Base Rate [Member] | Credit Facility [Member] | Circumstance Three [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   100.00%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   100.00%            
Base Rate [Member] | Credit Facility [Member] | Circumstance Four [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Interest rate (as a percent)   0.00%            
Base Rate [Member] | Credit Facility [Member] | Minimum [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   0.50%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   0.50%            
Base Rate [Member] | Credit Facility [Member] | Maximum [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   2.00%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   2.00%            
Federal Funds Rate [Member] | Credit Facility [Member] | Circumstance Two [Member] | 2017 Credit Facility [Member]                
Debt Instrument [Line Items]                
Basis points added to reference rate (as a percent))   50.00%            
Equipment Financing, Mortgages And Acquisition-Related Notes [Abstract]                
Basis points added to reference rate (as a percent)   50.00%