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Share Based Compensation (Schedule of Unvested Restricted Shares) (Detail) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Feb. 23, 2016
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period     134,733 345,344 341,710
Weighted Average Grant Date Fair Value          
Aggregate Fair Value Of Restricted Shares Granted To Employees On Grant Date     $ 4.0 $ 4.3 $ 4.1
Restricted Stock [Member]          
Shares          
Unvested at January 1 (in shares)   342,330 438,049 583,439 724,736
Shares granted     195,255 253,922 253,920
Shares vested     (282,125) (374,213) (392,917)
Shares forfeited     (8,849) (25,099) (2,300)
Unvested at December 31 (in shares)     342,330 438,049 583,439
Weighted Average Grant Date Fair Value          
Unvested at January 1 (in dollars per share)   $ 23.13 $ 19.11 $ 17.15 $ 14.81
Shares granted (in dollars per share)     23.38 19.20 18.54
Shares vested (in dollars per share)     17.12 16.16 13.74
Shares forfeited (in dollars per share)     21.32 18.46 16.41
Unvested at December 31 (in dollars per share)     $ 23.13 $ 19.11 $ 17.15
Subsequent Event [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period 226,521 125,447      
Weighted Average Grant Date Fair Value          
Aggregate Fair Value Of Restricted Shares Granted To Employees On Grant Date $ 4.2