Net Income Per Share - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net income allocated to PRS	$ 0.6	$ 0.7
Maximum [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Difference amount between basic and diluted net income per share	$ 0.01	$ 0.01

Amount of earnings (loss) distributed and earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 60B

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7655603&loc=SL5780133-109256

Name:	us-gaap_ParticipatingSecuritiesDistributedAndUndistributedEarningsLossBasic
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration