Notes Payable and Debt and Financing Arrangements - Letter of credit facilities and other sources of collateral (Details) $ in Thousands		Mar. 31, 2017 USD ($) Credit_Facilities	Dec. 31, 2016 USD ($) Credit_Facilities
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities		15	15
Letter of credit
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1]	$ 4,020,991	$ 3,991,687
Available letter of credit facilities - in use	[1],[2]	$ 2,430,003	$ 2,345,293
Collateralized by certain assets of the Company’s investment portfolio	[1]	49.10%	48.60%
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity		$ 712,300	$ 755,000
Available letter of credit facilities - commitments		1,000,000	1,000,000
Available letter of credit facilities - in use		$ 287,700	$ 245,000

[1] At March 31, 2017 and December 31, 2016, there were fifteen available credit facilities.

[2] At March 31, 2017 and December 31, 2016, the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.0 billion. However, at March 31, 2017 and December 31, 2016, $287.7 million and $245 million, respectively, were utilized under these facilities to issue letters of credit, leaving $712.3 million and $755.0 million, respectively, available either to issue additional letters of credit or to support other operating or financing needs under these particular facilities.