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Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Restricted and Deferred Stock Activity
A summary of restricted and deferred stock activity follows:
 
Number of shares
Non-vested shares outstanding at January 1, 2016
1,778,275

Awarded:

Time-vesting
127,167

Performance-based
137,374

Released:

Time-vesting
(414,389
)
Performance-based
(90,003
)
Forfeitures:
 
Time-vesting
(55,717
)
Performance-based
(161,415
)
Non-vested shares outstanding at December 31, 2016
1,321,292

Summary of Average Grant-Date Fair Value of Restricted Stock
The average grant-date fair value of restricted stock awarded in 2016, 2015 and 2014 follows:
 
2016
 
2015
 
2014
Time-vested
$
51.04

 
$
53.65

 
$
46.69

Performance-based
51.18

 
49.50

 
48.31

Schedule of Weighted Average Assumptions on Performance-Based Shares
The fair values of the performance-based shares awarded were calculated using a Monte Carlo valuation model and the following weighted average assumptions:
 
2016
 
2015
 
2014
Risk-free interest rate
1.2
%
 
1.1
%
 
0.8
%
Expected term (years)
3

 
3

 
3

Expected stock price volatility
19.8
%
 
17.4
%
 
21.5
%