Common Stock - Additional Information (Detail) (USD $)	3 Months Ended		12 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2015	Mar. 31, 2014	Mar. 10, 2015
Value of common shares planning to sell	$ 11,891,000
Net proceeds to Company	8,791,000	107,000	9,089,000	567,000
Common stock issued through Stock Incentive Plan, Restricted Stock/Unit Plan, and Management Incentive Plan	125,000
BNY Mellon Capital Markets, LLC [Member]
Common stock sold through agent	192,262
Common stock weighted average price	$ 57.47
Net proceeds to Company	10,938,956
Agent commissions	110,495
Common stock available for sale under the program	88,950,549		88,950,549
Maximum [Member]
Value of common shares planning to sell					$ 100,000,000