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Share-based compensation Additional Information (Details)
1 Months Ended 12 Months Ended
Mar. 01, 2014
shares
Feb. 28, 2013
shares
Dec. 31, 2015
USD ($)
increment
shares
Dec. 31, 2014
USD ($)
shares
Dec. 31, 2013
USD ($)
shares
Share-based compensation          
Income tax benefit (in dollars) | $     $ 2,300,000 $ 3,400,000 $ 2,800,000
Restricted shares          
Share-based compensation          
Fair value of vested stock (in dollars) | $       $ 100,000 $ 100,000
Shares vested | shares       4,503 4,502
Long-term incentive plan (LTIP)          
Share-based compensation          
Performance period     3 years    
Exception to forfeiture, minimum requisite service period     12 months    
Payout low end of range (as a percent)     0.00%    
Payout high end of range (as a percent)     200.00%    
Nonqualified stock options (NQSOs)          
Share-based compensation          
Exercisable (in shares) | shares     0    
Restricted stock units          
Share-based compensation          
Fair value of vested stock (in dollars) | $     $ 3,700,000 $ 4,100,000 $ 3,700,000
Income tax benefit (in dollars) | $     1,100,000 $ 1,200,000 $ 900,000
Unrecognized compensation cost | $     $ 3,900,000    
Weighted-average period over which unrecognized compensation cost expected to be recognized     2 years 5 months 24 days    
Shares vested | shares     102,173 144,702 118,885
LTIP linked to TRS          
Share-based compensation          
Fair value of vested stock (in dollars) | $     $ 0 $ 0 $ 2,200,000
Income tax benefit (in dollars) | $     0 $ 0 $ 900,000
Unrecognized compensation cost | $     $ 500,000    
Weighted-average period over which unrecognized compensation cost expected to be recognized     1 year    
Shares vested | shares     75,915 70,189 87,753
Shares settled | shares   70,205      
Expired (in shares) | shares   17,548      
LTIP awards linked to other performance conditions          
Share-based compensation          
Fair value of vested stock (in dollars) | $     $ 4,700,000 $ 1,900,000 $ 600,000
Income tax benefit (in dollars) | $     $ 1,800,000 $ 800,000 $ 200,000
Shares vested | shares     121,249 65,089 18,280
2012-2014 LTIP linked to other performance conditions          
Share-based compensation          
Unrecognized compensation cost | $     $ 1,000,000    
Weighted-average period over which unrecognized compensation cost expected to be recognized     1 year    
Equity and Incentive Plan (EIP)          
Share-based compensation          
Additional shares available for issuance | shares 1,500,000        
Shares remaining available for future issuance | shares     3,500,000    
Shares that can be issued upon vesting of outstanding units and achievement of performance goals | shares     500,000    
Equity and Incentive Plan (EIP) | Restricted shares          
Share-based compensation          
The number of equal annual increments for which the awards become unrestricted (in installments) | increment     4    
Stock Option and Incentive Plan (SOIP) | Stock appreciation rights (SARs)          
Share-based compensation          
Percentage of award exercisable in installments     25.00%    
Period during which the award vests and becomes exercisable in installments     4 years    
Period award expires if not exercised     10 years    
Nonemployee Director Stock Plan          
Share-based compensation          
Shares remaining available for future issuance | shares     141,044