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Share-Based Compensation Plans (Summary of Restricted Stock Unit Grants) (Details) - Restricted Stock [Member]
shares in Thousands
3 Months Ended
Mar. 31, 2018
$ / shares
shares
Summary of restricted stock activity  
Outstanding, beginning of period (in shares) | shares 779
Granted (in shares) | shares 225
Vested (in shares) | shares (404)
Forfeited (in shares) | shares (20)
Outstanding, end of period (in shares) | shares 580
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 $ 35.39
Granted (in usd per share) | $ / shares 75.18
Vested (in usd per share) | $ / shares 39.56
Forfeited (in usd per share) | $ / shares 42.04
Outstanding, end of period (in usd per share) | $ / shares $ 57.04