Stock-Based Compensation Plans (Summary of Activity Related to Restricted Stock Awards and Stock Purchase Grants) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Restricted Stock Units (RSUs) [Member]  
Number of Shares  
Nonvested, beginning 0
Granted 1,055
Vested (1,055)
Forfeited 0
Nonvested, ending 0
Weighted Average Grant Date Fair Value  
Nonvested, beginning $ 0.00
Granted $ 37.50
Vested $ 37.50
Forfeited $ 0.00
Nonvested, ending $ 0.00
Restricted Stock [Member]  
Number of Shares  
Nonvested, beginning 10,480
Granted 7,176
Vested (6,183)
Forfeited (516)
Nonvested, ending 10,957
Weighted Average Grant Date Fair Value  
Nonvested, beginning $ 34.53
Granted $ 39.06
Vested $ 35.47
Forfeited $ 34.57
Nonvested, ending $ 36.96
Management Stock Purchase Plan [Member]  
Number of Shares  
Nonvested, beginning 14,220
Granted 5,055
Vested (6,923)
Forfeited (247)
Nonvested, ending 12,105
Weighted Average Grant Date Fair Value  
Nonvested, beginning $ 11.14
Granted $ 12.35
Vested $ 10.55
Forfeited $ 11.43
Nonvested, ending $ 11.97