Notes Payable and Debt and Financing Arrangements - Letter of credit facilities and other sources of collateral (Details) $ in Thousands		Aug. 05, 2016 USD ($)	Jun. 30, 2016 USD ($) Credit_Facilities	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]		$ 4,488,072	$ 4,463,041
Available letter of credit facilities - in use			$ 2,486,602	$ 2,515,653	[1],[2]
Collateralized by certain assets of the Company’s investment portfolio	[1]		51.40%	50.90%
Revolving credit facility
Line of Credit Facility [Line Items]
Available credit facilities - remaining borrowing capacity			$ 848,800	$ 822,900
Available letter of credit facilities - commitments			1,400,000	1,400,000
Available letter of credit facilities - in use			$ 501,200	$ 527,100
Subsequent Event [Member] | Two Thousand Sixteen Credit Agreements [Member]
Line of Credit Facility [Line Items]
Line Of Credit Facility Optional Increased Capacity		$ 500,000
Subsequent Event [Member] | 2016 Secured Credit Agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1]	750,000
Line Of Credit Facility Optional Increased Capacity		250,000
Subsequent Event [Member] | 2016 Unsecured Credit Agreement
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	[1]	750,000
Line Of Credit Facility Optional Increased Capacity		$ 250,000

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

[2] As of June 30, 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 June 30, 2016 and December 31, 2015, $501.2 million and $527.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $848.8 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.