Notes Payable and Debt and Financing Arrangements - Letter of credit facilities and other sources of collateral (Details) $ in Thousands		Sep. 30, 2016 USD ($) Credit_Facilities	Aug. 05, 2016 USD ($)	Dec. 31, 2015 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]	$ 3,991,806		$ 4,463,041
Available letter of credit facilities - in use	[1],[2]	$ 2,482,386		$ 2,515,653
Collateralized by certain assets of the Company’s investment portfolio	[1]	51.10%		50.90%
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity		$ 598,500		$ 822,900
Available letter of credit facilities - commitments		1,100,000		1,400,000
Available letter of credit facilities - in use		$ 501,500		$ 527,100
Two Thousand Sixteen Credit Agreements [Member]
Line of Credit Facility [Line Items]
Line Of Credit Facility Optional Increased Capacity			$ 500,000
2016 Secured Credit Agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1]		750,000
2016 Unsecured Credit Agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1]		$ 750,000

[1] As of September 30, 2016 and December 31, 2015, there were fifteen available credit facilities

[2] As of September 30, 2016 and December 31, 2015, the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.1 billion and $1.4 billion, respectively. However, as of September 30, 2016 and December 31, 2015, $501.5 million and $527.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $598.5 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.