Notes Payable and Debt and Financing Arrangements - Letter of credit facilities and other sources of collateral (Details) $ in Thousands		Mar. 31, 2016 USD ($) Credit_Facilities	Dec. 31, 2015 USD ($)
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities		15
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity		$ 823,500	$ 822,900
Available letter of credit facilities - commitments		1,400,000
Available letter of credit facilities - in use		526,500	527,100
Letter of credit facilities
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1]	4,503,617	4,463,041
Available letter of credit facilities - in use		$ 2,548,085	$ 2,515,653	[1],[2]
Collateralized by certain assets of the Company’s investment portfolio	[1]	51.30%	50.90%

[1] As of March 31, 2016 and December 31, 2015, there were fifteen available credit facilities

[2] As of March 31, 2016 and December 31, 2015 the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.4 billion. However, as of March 31, 2016 and December 31, 2015, $526.5 million and $527.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $823.5 million and $822.9 million, respectively, available either to issue additional letters of credit or to support other operating or financing needs under these particular facilities.