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Convertible Subordinated Notes (Narrative) (Details)
 3 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
Mar. 19, 2014
component
$ / shares
Dec. 31, 2016
USD ($)
$ / shares
Sep. 30, 2016
$ / shares
Jun. 30, 2016
$ / shares
Mar. 31, 2016
$ / shares
Sep. 30, 2014
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]                    
Common Stock, Dividends, Per Share, Cash Paid | $ / shares     $ 0.050 $ 0.050 $ 0.025 $ 0.025   $ 0.075 $ 0.05  
Debt instrument maturity date             Mar. 15, 2021      
Deferred tax liability $ 225,000   $ 615,000         $ 225,000 $ 615,000  
Amortization of debt issuance costs               820,000 824,000 $ 921,000
Deferred Tax Liabilities, Convertible Subordinated Notes 4,096,000   $ 8,636,000         4,096,000 8,636,000  
Convertible subordinated notes                    
Debt Instrument [Line Items]                    
Principal amount $ 143,750,000.00             $ 143,750,000.00    
Debt instrument interest rate (in percent)   2.75%                
Debt conversion (in dollars per share) | $ / shares $ 22.41 $ 22.56           $ 22.41    
Debt redemption price (percent)   100.00%                
Debt instrument, term               7 years    
Number of recognition components, convertible debt | component   2                
Deferred tax liability $ 12,700,000             $ 12,700,000    
Conversion ratio 44.6266 44.3169           0.0444108    
Long-term debt, fair value $ 180,300,000             $ 180,300,000    
Interest expense               4,000,000    
Amortization of debt issuance costs               500,000    
Accretion of discount               $ 4,300,000 $ 3,900,000  
Remaining discount amortization period               38 months    
Effective interest rate (percent) 2.75%   6.75%         2.75% 6.75%