Property and equipment: (Details) (USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Property and equipment
Depreciation and amortization expense	$ 15,874,000	$ 15,239,000
Capitalized salaries and related benefits of employees	1,900,000	1,700,000
Property, equipment and capital leases
Property and equipment
Depreciation and amortization expense	$ 15,800,000	$ 15,200,000

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6943989&loc=d3e3602-108585

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