Notes Payable and Debt and Financing Arrangements (Details) - Letter of credit summary (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012 LOCs	Dec. 31, 2011
Line of Credit Facility [Line Items]
Available credit facilities - commitments		$ 3,000,000
Collateralized by certain assets of the Company's investment portfolio	93.30%	93.80%
Number of letter of credit facilities available	5
Letter of credit facility
Line of Credit Facility [Line Items]
Available credit facilities - commitments	4,000,000	4,000,000
Available letter of credit facilities - in use	1,757,250	1,871,192
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - commitments	1,000,000	$ 1,000,000

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 Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Reference 2: 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 3: 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