Vacation Pay - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Executive Compensation Plan Expense [Line Items]
Unutilized earned leave expense	$ 2.2	$ 1.6
Accrual for unutilized earned leave	$ 18.9		$ 17.9

Amount of liability accrued for the employer's obligation related to an employee's right to receive compensation for future absences for vacation and holidays that have been earned by employee. This amount does not include any amounts accrued for sabbatical leave.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 710

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6409733&loc=d3e19366-108361

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 43

-Paragraph 6

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_CompensatedAbsencesLiabilityVacationAndHoliday
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant