Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	0 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Mar. 15, 2013 Unsecured Debt [Member] 5.125% unsecured notes [Member]	Jun. 30, 2013 Unsecured Debt [Member] 5.125% unsecured notes [Member]
Debt Instrument [Line Items]
Repayments of Unsecured Debt	$ 60,000	$ 50,000	$ 60,000
Interest rate, stated percentage			5.125%	5.125%

The cash outflow to repay long-term debt that is not secured by collateral. Excludes repayments of tax exempt unsecured debt.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3291-108585

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

Name:	us-gaap_RepaymentsOfUnsecuredDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration