Commitments and Contingencies, Letters of Credit (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 80,000,000
Letters of Credit Outstanding, Amount	$ 19,400,000	$ 20,800,000

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

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