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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
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Granted 279,890 231,830 365,831
Performance Based Shares      
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward      
Beginning Balance 495,731 628,009 585,247
Granted 344,488 323,814 365,831
Quantity adjusted by TSR factor 39,566    
Vested (487,784) (430,646) (311,078)
Forfeited (8,019) (25,446) (11,991)
Ending Balance 383,982 495,731 628,009
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 $ 21.16
Granted 25.86 33.37 29.86
Quantity adjusted by TSR factor 24.37    
Vested 27.72 30.45 19.02
Forfeited 29.13 28.65 29.33
Non-vested at end of period $ 26.35 $ 28.25 $ 27.13