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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 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward          
Granted 118,390 231,830 279,890 231,830  
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     344,488   323,814
Quantity adjusted by TSR factor     10,315    
Vested     (403,654)   (430,646)
Forfeited         (25,446)
Ending Balance 446,880   446,880   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     25.86   33.37
Quantity adjusted by TSR factor     26.45    
Vested     27.09   30.45
Forfeited         28.65
Non-vested at end of period $ 26.86   $ 26.86   $ 28.25