Note Purchase and Private Shelf Agreement - Additional Information (Detail) (3.68% Series B Senior Notes Mature on March 17, 2021 [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Mar. 17, 2014
3.68% Series B Senior Notes Mature on March 17, 2021 [Member]
Debt Instrument [Line Items]
Aggregate principal amount		$ 40.0
Senior Notes, interest rate		3.68%
Debt instrument, payment terms	Interest on the Notes is payable semi-annually beginning on September 17, 2014 and continuing thereafter on March 17 and September 17 of each year until maturity.
Debt instrument, maturity date	Mar. 17, 2021