Summary of restricted stock activity under the incentive plan for members of the Board of Directors (Detail) (Restricted Stock Units RSU Member, Director Member, USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Restricted Stock Units RSU Member | Director Member
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward
Beginning Balance			0	0	0
Granted	2,318	1,669	21,051	18,885	20,930
Vested			(21,051)		(20,930)
Forfeited			0		0
Ending Balance	0		0		0
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Additional Disclosures Abstract
Non-vested at beginning of period			$ 0	$ 0	$ 0
Granted			$ 30.26		$ 29.43
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value			$ 30.26		$ 29.43
Forfeited			$ 0		$ 0
Non-vested at end of period	$ 0		$ 0		$ 0