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Share-Based Compensation Plans (Summary of Performance-Based Share Grants) (Details) - Total Shareholder Return Performance-Based Shares - Performance Based Shares
shares in Thousands
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Summary of performance share activity  
Outstanding, beginning of period (in shares) | shares 213
Granted (in shares) | shares 76
Forfeited (in shares) | shares (8)
Outstanding, end of period (in shares) | shares 281
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]  
Outstanding, beginning of period (in usd per share) | $ / shares $ 47.04
Granted (in usd per share) | $ / shares 99.24
Forfeited (in usd per share) | $ / shares 45.50
Outstanding, end of period (in usd per share) | $ / shares $ 61.15