Credit Agreements (Details) ([UnsecuredSyndicatedCreditFacilityBMember], USD $)	6 Months Ended	9 Months Ended
Jun. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
[UnsecuredSyndicatedCreditFacilityBMember]
Credit Agreements Details [Line Items]
Modification date	18-Apr-13	15-Jun-13
Total amount of credit facilities available	$ 50,000,000	$ 0	$ 500,000,000
Line Of Credit Facility Revolving Credit Amount	$ 50,000,000	$ 0	$ 375,000,000

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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