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Note Stock-based compensation (Summary of restricted stock activity under the incentive plan for members of management) (Detail) - Restricted Stock Units (RSUs) - Executive Officers - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Granted 161,500 0  
Performance Based Shares      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 495,731 628,009 628,009
Granted 226,098   323,814
Quantity adjusted by TSR factor (8,914)    
Vested (256,501)   (430,646)
Forfeited     (25,446)
Ending Balance 456,414   495,731
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract      
Non-vested at beginning of period $ 28.25 $ 27.13 $ 27.13
Granted 24.05   33.37
Quantity adjusted by TSR factor 28.47    
Vested 25.9   30.45
Forfeited     28.65
Non-vested at end of period $ 27.12   $ 28.25