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Stock Compensation Plans - Significant Assumptions and Historical Vesting for TSR Performance Stock Awards (Details 6) - Performance stock awards - Total Stockholder Return
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Significant assumptions used      
Expected dividend yield (as a percent) 0.96% 0.00% 0.00%
Expected volatility (as a percent) 25.90% 31.50% 37.80%
Risk-free interest rates (as a percent) 0.93% 0.68% 0.34%
Performance (vesting) period 3 years 3 years 3 years