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Stock compensation plans (Tables)
 12 Months Ended
Dec. 31, 2014
Share-based Compensation [Abstract]  
Weighted-Average Assumptions used to Estimate Fair Value of Options Granted
The fair value of options granted in 2014, 2013 and 2012 was estimated at the date of grant using a multiple point Black-Scholes option pricing model. The following weighted-average assumptions were used:
 
 
2014
 
2013
 
2012
Risk-free interest rate
1.45
%
 
0.75
%
 
0.81
%
Expected life of option
4.89 years

 
4.87 years

 
4.85 years

Expected dividend yield
1.34
%
 
1.73
%
 
2.28
%
Expected volatility
21.44
%
 
24.65
%
 
28.46
%
Weighted-Average Assumptions used to Estimate Fair Value of Non-Vested Shares Granted
The following weighted-average assumptions were used:
 
 
2014
 
2013
 
2012
Risk-free interest rate
0.65
%
 
0.36
%
 
0.37
%
Expected dividend yield
1.34
%
 
1.71
%
 
2.24
%
Summary of Stock Option Activity
The following table summarizes the option activity during 2014:
 
 
Shares Subject to Options
 
Weighted Average Exercise Price
 
Weighted Average Remaining Contractual Life In Years
 
Aggregate
Intrinsic
Value
 
 
 
 
 
 
 
(Dollars in thousands)
Outstanding, beginning of the year
1,279,480

 
$
65.05

 
 
 
 
Granted
343,580

 
101.45

 
 
 
 
Exercised
(362,719
)
 
60.86

 
 
 
 
Forfeited or expired
(26,669
)
 
87.37

 
 
 
 
Outstanding, end of the year
1,233,672

 
75.93

 
7.3
 
$
47,974

Exercisable, end of the year
647,425

 
$
64.82

 
6.1
 
$
32,373
Summary of Non-Vested Restricted Stock Unit Activity
The Company recorded $4.6 million of expense related to the portion of the shares underlying options