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9. Share-based Compensation (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value $ 2,100,000 $ 2,200,000 $ 2,100,000  
Share-based Compensation Expense $ 19,700,000 $ 17,000,000.0 $ 14,900,000  
2011 Employee Stock Purchase Plan [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares Authorized 600,000      
Stock Option and Award Plans [Member] | Incentive Award Plan (2007) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares Authorized 2,250,360      
Number of Shares Available for Grant 587,509      
Stock Options [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options Granted Term P10Y0M0D      
Weighted Average Remaining Contractual Term (in years) - Outstanding 5 years 1 month 24 days      
Aggregate Intrinsic Value (in millions) - Outstanding $ 34.1      
Options - Weighted-Average Exercise Price $ 106.52 $ 100.36 $ 84.50 $ 78.72
Options Granted - Weighted Average Exercise Price 159.37 139.56 119.71  
Options Exercised - Weighted Average Exercise Price 80.70 61.29 63.66  
Options Forfeitured/expired - Weighted Average Exercise Price $ 127.65 $ 62.47 $ 100.29  
Options - Shares Vested and Expected to Vest 437,350      
Options Vested and Expected to Vest - Weighted Average Exercise Price $ 105.39      
Options Vested and Expected to Vest - Weighted Average Remaining Contractual Term (in years) 5 years 14 days      
Options Vested and Expected to Vest - Aggregate Intrinsic Value (in millions) $ 33.6      
Weighted Average Exercise Price - Options Exercisable $ 93.78      
Weighted Average Remaining Contractual Term (in years) - Exercisable 3 years 9 months 18 days      
Options Exercisable Aggregate Intrinsic Value (in millions) $ 27.8      
Options, Exercises in Period, Total Intrinsic Value 1,000,000 $ 13,000,000 $ 5,000,000  
Cash Received from Exercise of Stock Options 1,200,000 2,900,000 5,800,000  
Tax Benefit Realized from Exercise of Stock Options 6,000,000.0 $ 9,300,000 $ 5,300,000  
Total unrecognized compensation cost from stock options $ 4,900,000      
Options Number Exercisable 314,330      
Options Vesting Period 5 years      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 20.00%      
Weighted average fair value of options granted, period for recognition 3 years      
Expected Volatility 21.00% 23.00% 25.00%  
Risk Free Interest Rate 1.35% 1.90% 2.35%  
Expected life (in years) 7 years 4 months 24 days 7 years 8 months 12 days 8 years 8 months 12 days  
Expected dividend $ 0 $ 0 $ 0  
Weighted average fair value of options granted $ 42.40 $ 42.74 $ 43.96  
Expected dividend yield 0.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Outstanding beginning of period 426,705 581,806 629,643  
Options - Shares Granted 45,000 40,500 54,000  
Options - Shares Exercised (15,285) (195,221) (91,387)  
Options - Shares Forfeitures/expired (6,850) (380) (10,450)  
Outstanding end of period 449,570 426,705 581,806  
Employee Stock [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Number of Shares Available for Grant 96,121      
Cash Received from Exercise of Stock Options $ 11,500,000 $ 10,800,000 $ 10,200,000  
Stock Issued During Period, Shares, Employee Stock Purchase Plans 93,605 96,634 102,222  
Employee Contribution Rate - Maximum 10.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Employee Contribution Amount $ 25,000      
Employee Purchase Price Discount from Market Price 85.00%      
Expected Volatility 20.00% 18.00% 15.00%  
Risk Free Interest Rate 0.26% 0.02% 0.04%  
Expected life (in years) 7 days 7 days 7 days  
Expected dividend $ 0 $ 0 $ 0  
Expected dividend yield 0.00%      
Share-based Compensation Arrangement by Share-based Payment Award, Per Share Weighted Average Price of Shares Purchased $ 27.36 $ 25.08 $ 21.88  
Stock Award Plans [Member] | Incentive Award Plan (2007) [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Options Vesting Period 5 years      
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage 20.00%