Notes Payable and Debt and Financing Arrangements (Detail) - Schedule of Letter of Credit Facilities (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Collateralized by certain of the Company’s investment portfolio	93.40%	[1]	93.80%	[1]
Letter of Credit [Member]
Letter credit facilities	4,000,000	[1],[2]	4,000,000	[1],[2]
Available letter of credit facilities - in use	1,774,251	[1]	1,871,192	[1]
Revolving Credit Facilities [Member]
Letter credit facilities	1,000,000	[1],[3]	1,000,000	[1],[3]

[1] At March 31, 2012 and December 31, 2011 there were five available letter of credit facilities.

[2] The Company has the option to increase the size of the March 2011 Credit Agreement by an additional $500 million and the size of the facilities under the December 2011 Credit Agreements by an additional $500 million across both such facilities.

[3] At March 31, 2012 and December 31, 2011 the revolving credit facility was unutilized.