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Compensation Plans (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2018
USD ($)
$ / shares
shares
Jun. 30, 2018
shares
Sep. 30, 2017
USD ($)
Jun. 30, 2017
shares
Sep. 30, 2018
USD ($)
shares
$ / shares
Sep. 30, 2017
USD ($)
shares
Dec. 31, 2017
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 5,700,000 5,700,000       5,700,000    
Employee Stock Purchase Plan [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription Rate 15.00% 15.00%       15.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Subscription | $ $ 25,000 $ 25,000       $ 25,000    
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent           85.00%    
Stock Issued During Period, Shares, Employee Stock Purchase Plans           100,249 123,678  
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Excluding Stock Options | $           $ 1,900,000 $ 1,700,000  
Director [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Stock Issued During Period, Shares, Share-based Compensation, Gross     58,572   71,573      
Performance Shares [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Multiplier applied to PSU awards at settlement 0 0       0    
Stock-based compensation expense | $   $ 3,000,000   $ 2,600,000   $ 7,700,000 6,800,000  
Unrecognized stock based compensation expense | $ $ 22,900,000 $ 22,900,000       $ 22,900,000    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number [1] 1,776,191 1,776,191       1,776,191   1,533,491
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares $ 20.66 $ 20.66       $ 20.66   $ 22.97
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period [1]           572,924    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares           $ 24.45    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period [1]           (233,102)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value | $ / shares           $ 44.25    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period [1]           (97,122)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value | $ / shares           $ 22.89    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Multiplier Assumed           1    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value | $           $ 14,000,000    
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period           0    
Restricted Stock Units (RSUs) [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years              
Stock-based compensation expense | $   $ 3,000,000   $ 2,900,000   $ 8,000,000 $ 7,500,000  
Unrecognized stock based compensation expense | $ $ 24,100,000 $ 24,100,000       $ 24,100,000    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number 1,308,144 1,308,144       1,308,144   1,244,262
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value | $ / shares $ 21.46 $ 21.46       $ 21.46   $ 20.25
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period           583,552    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | $ / shares           $ 25.77    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period           (407,529)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value | $ / shares           $ 24.30    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period           (112,141)    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value | $ / shares           $ 17.93    
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Granted in Period, Total Fair Value | $           $ 15,000,000    
Number of shares represented by each RSU 1              
Stock Issued During Period, Shares, Share-based Compensation, Gross           407,529    
Restricted Stock, Shares Issued Net of Shares for Tax Withholdings           291,745    
Restricted Stock Units (RSUs) [Member] | Director [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period         8,794      
Minimum [Member] | Performance Shares [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Multiplier applied to PSU awards at settlement 0 0       0    
Maximum [Member] | Performance Shares [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Multiplier applied to PSU awards at settlement 2 2       2    
Performance Period [Member] | Performance Shares [Member]                
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years              
[1] (1) The number of awards assumes a multiplier of one. The final number of shares of common stock issued may vary depending on the three-year performance multiplier which ranges from zero to two.