Notes Payable and Debt and Financing Arrangements - Letter of credit summary (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1],[2]	$ 3,991,687	$ 4,463,041
Available letter of credit facilities - in use	[2]	$ 2,345,293	$ 2,515,653
Collateralized by certain assets of the Company's investment portfolio	[2]	48.60%	50.90%
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments		$ 1,000,000	$ 1,400,000
Available letter of credit facilities - in use		$ 245,000	$ 527,100

[1] As of December 31, 2016 and December 31, 2015 the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.0 billion and $1.4 billion, respectively. However, as of December 31, 2016 and December 31, 2015, $245.0 million and $527.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $755.0 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.

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