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Stock Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2015
Share-based Compensation [Abstract]  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
The assumptions used to value the officer LTIP awards were as follows:
 
2015 Awards
 
2014 Awards
Expected volatility (1)
17.2 - 17.5%
 
23.2
%
Risk-free interest rate (2)
1.0 - 1.1%
 
0.8
%
Expected term (3)
3 and 4 years
 
3 and 4 years

Share price at grant date
$27.66 - $27.76
 
                          $24.08
(1) Expected volatility based upon historical volatility of our daily closing share price.
(2) Risk-free interest rate based on U.S. treasury constant maturity bonds on the measurement date with a maturity equal to the market condition performance period.
(3) Expected term based on the market condition performance period.
Schedule of Restricted Share Awards [Table Text Block]
The activity for the three years ended December 31, 2015 related to our restricted share awards, excluding those subject to market conditions, was as follows:
 
Shares
 
Wtd Avg Grant Fair Value
Unvested at December 31, 2012
149,803

 
$
27.37

Granted
141,609

 
26.30

Vested during year
(158,657
)
 
26.66

Forfeited
(2,940
)
 
27.80

Unvested at December 31, 2013
129,815

 
27.06

Granted
210,817

 
23.93

Vested during year
(236,498
)
 
25.06

Forfeited
(10,467
)
 
25.80

Unvested at December 31, 2014
93,667

 
25.22

Granted
251,642

 
27.80

Vested during year
(212,856
)
 
27.18

Forfeited
(26,309
)
 
26.77

Unvested at December 31, 2015
106,144

 
27.71
Performance Share Units with Market Conditions [Table Text Block]
Stock based awards with market conditions under the LTIP were granted in 2015 and 2014 with fair market values, as determined using a Monte Carlo simulation, as follows (in thousands):
 
Grant Date Fair Value
 
2015 Awards
 
2014 Awards
 
Restricted
 
Unrestricted
 
Restricted
 
Unrestricted
Relative TSR
$
191

 
$
634

 
$
458

 
$
1,376

Absolute TSR
76

 
254

 
327

 
921
Schedule Of Unamortized Value Of Awards With Market Conditions [Table Text Block]
The unamortized value of these awards with market conditions as of December 31, 2015 was as follows (in thousands):
 
2015 Awards
 
2014 Awards
 
Restricted
 
Unrestricted
 
Restricted
 
Unrestricted
Relative TSR
$
144

 
$
383

 
$
161

 
$
—

Absolute TSR
57

 
153

 
111

 
—