Nonvested Restricted Shares (Detail) (USD $)	3 Months Ended				12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Nonvested, Weighted Average Grant Date Fair Value, Beginning of Period	$ 34.90	$ 34.86	$ 34.88	$ 31.69	$ 31.69
Granted		$ 46.91	$ 38.90	$ 36.28	$ 36.40	$ 30.06	$ 45.38
Vested	$ 34.93	$ 35.58	$ 35.40	$ 29.54
Forfeited		$ 0.00	$ 0.00	$ 0.00
Nonvested, Weighted Average Grant Date Fair Value, End of Period	$ 34.90	$ 34.90	$ 34.86	$ 34.88	$ 34.90	$ 31.69
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested Restricted Shares, Beginning Balance	370,394	384,426	399,489	348,918	348,918
Nonvested Restricted Shares, Granted		2,223	226	204,731
Restricted Shares, Vested	(2,126)	(16,255)	(15,289)	(154,160)
Nonvested Restricted Shares, Forfeited		0	0	0
Nonvested Restricted Shares, Ending Balance	368,268	370,394	384,426	399,489	368,268