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Share-Based Compensation Plans - Summary of information about stock options (Details) - $20.42 - $24.48
12 Months Ended
Dec. 31, 2017
$ / shares
shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]  
Range of Exercise Prices, Lower Range $ 24.48
Range of Exercise Prices, Upper Range $ 24.48
Stock Options Outstanding and Exercisable, Number (in shares) | shares 93,000
Stock Options Outstanding and Exercisable, Weighted Average Remaining Life 2 years 5 months 12 days
Stock Options Outstanding and Exercisable, Weighted Average Exercise Price $ 24.48