Summary of restricted stock activity under the incentive plan for members of the Board of Directors (Detail) - Restricted Stock Units RSU Member - Director Member - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward
Beginning Balance			0	0	0
Granted	15,386	15,648	18,029	18,733	23,135
Vested			(18,029)		(23,135)
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			32.78		30.43
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Vested In Period Weighted Average Grant Date Fair Value			32.78		30.43
Forfeited			0		0
Non-vested at end of period	$ 0		$ 0		$ 0