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Benefit Plans - Summary of Company's Restricted Stock Units Activity (Detail) (Restricted Stock Units [Member], USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Restricted Stock Units [Member]
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of Shares, Beginning Balance 262,820 187,810 110,200
Number of Shares, Granted 150,300 109,200 125,800
Number of Shares, Vested (87,840) (34,190) (39,105)
Number of Shares, Cancelled/Forfeited/Expired (4,300)    (9,085)
Number of Shares, Ending Balance 320,980 262,820 187,810
Weighted-Average Grant Date Fair Value, Beginning Balance $ 28.22 $ 26.33 $ 23.61
Weighted-Average Grant Date Fair Value, Granted $ 26.89 $ 31.37 $ 27.99
Weighted-Average Grant Date Fair Value, Vested $ 24.98 $ 27.85 $ 23.91
Weighted-Average Grant Date Fair Value, Cancelled/Forfeited/Expired $ 29.24    $ 25.36
Weighted-Average Grant Date Fair Value, Ending Balance $ 28.47 $ 28.22 $ 26.33
Aggregate Intrinsic Value $ 12.8