Notes Payable and Debt and Financing Arrangements - Letter of credit summary (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1],[2]	$ 4,463,041	$ 3,575,000
Available letter of credit facilities - in use	[2]	$ 2,515,653	$ 1,790,561
Collateralized by certain assets of the Company's investment portfolio	[2]	50.90%	66.20%
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		$ 1,400,000	$ 1,600,000
Available letter of credit facilities - in use		$ 527,100	$ 606,100

[1] As of December 31, 2015 and December 31, 2014 the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.4 billion and $1.6 billion, respectively. However, as of December 31, 2015 and December 31, 2014, $527.1 million and $606.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $822.9 million and $968.9 million, respectively, available either to issue additional letters of credit or to support other operating or financing needs under these particular facilities.

[2] As of December 31, 2015 and December 31, 2014, there were fifteen and eight available credit facilities, respectively.