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Stock compensation plans (Tables)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation [Abstract]    
Weighted-Average Assumptions used to Estimate Fair Value of Options Granted  
The fair value of options granted in 2017, 2016 and 2015 was estimated at the date of grant using a Black-Scholes option pricing model. The following weighted-average assumptions were used:
 
2017
 
2016
 
2015
Risk-free interest rate
1.88
%
 
1.30
%
 
1.44
%
Expected life of option
4.94 years

 
4.91 years

 
4.87 years

Expected dividend yield
0.71
%
 
0.94
%
 
1.12
%
Expected volatility
21.74
%
 
21.64
%
 
20.68
%
Weighted-Average Assumptions used to Estimate Fair Value of Non-Vested Shares Granted
The fair value for stock awards granted in 2017, 2016 and 2015 was estimated at the date of grant based on the market price for the underlying stock on the grant date discounted for the risk free interest rate and the present value of expected dividends over the vesting period. The following weighted-average assumptions were used:
 
2017
 
2016
 
2015
Risk-free interest rate
1.47
%
 
0.94
%
 
0.94
%
Expected dividend yield
0.71
%
 
0.93
%
 
1.12
%
  
Summary of Stock Option Activity
The following table summarizes the option activity during 2017:
 
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
1,607,745

 
$
99.51

 
 
 
 
Granted
260,171

 
191.42

 
 
 
 
Exercised
(144,995
)
 
94.83

 
 
 
 
Forfeited or expired
(13,993
)
 
148.94

 
 
 
 
Outstanding, end of the year
1,708,928

 
113.49

 
6.4
 
$
231,261

Exercisable, end of the year
1,168,768

 
$
91.21

 
5.4
 
$
184,206

  
Summary of Non-Vested Restricted Stock Unit Activity
The following table summarizes the non-vested restricted stock unit activity during 2017:
 
Number of
Non-Vested
Shares
 
Weighted
Average
Grant-Date
Fair Value
 
Weighted
Average
Remaining
Contractual
Life
 
Aggregate
Intrinsic
Value
 
 
 
 
 
(In years)
 
(Dollars in thousands)
Outstanding, beginning of the year
250,389

 
$
119.44

 
 
 
 
Granted
82,865

 
187.85

 
 
 
 
Vested
(85,271
)
 
102.66

 
 
 
 
Forfeited
(14,241
)
 
136.20

 
 
 
 
Outstanding, end of the year
233,742

 
148.79

 
1.2
 
$
58,160