Notes Payable and Debt and Financing Arrangements - Letter of credit summary (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 30, 2014
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 3,575,000	[1],[2]	$ 3,575,000	[1],[2]
Available letter of credit facilities - in use	1,790,561	[1],[3]	1,895,425	[1],[3]
Collateralized by certain assets of the Company's investment portfolio	66.20%	[1]	67.60%	[1]
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	1,000,000	[1],[4]	1,000,000	[1],[4]
Available letter of credit facilities - in use	0	[3]	0	[3]
ING Bank N.V., London Branch [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	150,000
Citibank Europe plc [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	$ 600,000				$ 750,000

[1] At December 31, 2014, there were eight available letter of credit facilities; at December 31, 2013, there were seven available letter of credit facilities.

[2] The Company has the option to increase the size of the facilities under the Syndicated Credit Agreements by an additional $500 million across such facilities. The Company also has the option to increase the maximum amount of the letters of credit and revolving credit loans available under the 2013 Citi Agreements, with the lender's and issuing lender's consent.

[3] “In Use” and “Outstanding” data represent December 31, 2014 and 2013 accreted values.

[4] At December 31, 2014 and 2013, the revolving credit available under the Syndicated Credit Agreements (as defined below) and under the December 9, 2011 unsecured credit agreement, respectively, was unutilized. The 2013 Citi Agreements (as defined below) provide for issuance of letters of credit and revolving credit loans in an aggregate amount of up to $575 million. At December 31, 2014, $575 million of letters of credit were issued under the 2013 Citi Agreements and therefore such amount is not included here.