Borrowing Arrangements - Summary of Unsecured Credit Facility (Detail)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Debt Disclosure [Abstract]
Weighted average rates	2.44%us-gaap_DebtWeightedAverageInterestRate	2.99%us-gaap_DebtWeightedAverageInterestRate
Range of effective rates, minimum	1.87%	1.88%
Range of effective rates, maximum	4.00%	3.91%

When presenting a range of interest rates, the highest effective rate for funds borrowed under the debt agreement as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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