Notes Payable and Debt and Financing Arrangements - Letter of credit summary (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 3,392,986	$ 3,991,687
Available letter of credit facilities - in use	$ 2,549,226	$ 2,345,293
Collateralized by certain assets of the Company's investment portfolio	54.20%	48.60%
Revolving credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	$ 750,000	$ 1,000,000
Available letter of credit facilities - in use	2,000	245,000
Citibank N.A. [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	0	250,000
Available letter of credit facilities - in use	0	245,000
Twenty Fifteen Goldman Sachs Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	0	200,000
Available letter of credit facilities - in use	0	200,000
Credit Agricole Facility I [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Credit Agricole Facility II [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Commonwealth Bank Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	215,000	0
Available letter of credit facilities - in use	215,000	0
Credit Suisse Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	100,000	0
Available letter of credit facilities - in use	100,000	0
Funds at Lloyds Bilateral Facility I [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Funds at Lloyds Bilateral Facility II [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Funds at Lloyds Bilateral Facility III [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Funds at Lloyds Bilateral Facility IV [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	125,000	125,000
Available letter of credit facilities - in use	125,000	125,000
Syndicated Lenders [Member] | Unsecured Debt [Member]
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	750,000	750,000
Available letter of credit facilities - in use	2,000	0
Commerzbank Facility [Member] | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	100,000	0
Available letter of credit facilities - in use	100,000	0
Total unsecured LOC facilities | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	1,915,000	1,950,000
Available letter of credit facilities - in use	1,167,000	1,195,000
Facilities collateralized by certain investment assets | Letter of credit facility
Line of Credit Facility [Line Items]
Available letter of credit facilities - commitments	1,477,986	2,041,687
Available letter of credit facilities - in use	$ 1,382,226	$ 1,150,293