Notes Payable and Debt and Financing Arrangements - Narrative relating to credit facilities (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) Credit_Facilities	Dec. 31, 2016 USD ($) Credit_Facilities	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) Credit_Facilities
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities	10	9
Credit Agricole Facility II [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 125,000,000
Funds at Lloyds Bilateral Facility [Member]
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities				4
Available letter of credit facilities - commitments				$ 500,000,000
Funds at Lloyds Bilateral Facility I [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments				125,000,000
Funds at Lloyds Bilateral Facility II [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000,000
Funds at Lloyds Bilateral Facility III [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000,000
Funds at Lloyds Bilateral Facility IV [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000,000
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	2,000,000	$ 245,000,000
Available letter of credit facilities - commitments	750,000,000	1,000,000,000
Line of credit facility, remaining borrowing capacity	748,000,000	755,000,000
Syndicated Credit Agreements [Member]
Line of Credit Facility [Line Items]
Optional increase capacity			$ 500,000,000
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	2,549,226,000	2,345,293,000
Available letter of credit facilities - commitments	3,392,986,000	$ 3,991,687,000
Credit Agricole Facility I [Member]
Line of Credit Facility [Line Items]
Number of letter of credit facilities available | Credit_Facilities		2
Available letter of credit facilities - commitments		$ 125,000,000
Catlin Group Limited [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use				$ 4
Syndicated Lenders [Member] | Secured Debt [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments			$ 750,000,000
Twenty Fifteen Goldman Sachs Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	0	200,000,000
Available letter of credit facilities - commitments	0	200,000,000
Citibank N.A. [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	0	245,000,000
Available letter of credit facilities - commitments	0	250,000,000
Commonwealth Bank Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	215,000,000	0
Available letter of credit facilities - commitments	215,000,000	0
Credit Suisse Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	100,000,000	0
Available letter of credit facilities - commitments	100,000,000	0
Commerzbank Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - in use	100,000,000	0
Available letter of credit facilities - commitments	$ 100,000,000	$ 0
Maximum | Credit Agricole Facility I [Member]
Line of Credit Facility [Line Items]
Credit Facility Required Notice Period	90 days
Minimum | Credit Agricole Facility I [Member]
Line of Credit Facility [Line Items]
Credit Facility Required Notice Period	30 days