Summary of Significant Accounting Policies - Concentration of Credit Risk (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Wells Fargo Funding, Inc. [Member]
Concentration of credit risk	58.00%	52.00%	44.00%
Bank of America, N.A. [Member]
Concentration of credit risk	0.00%	17.00%	38.00%
JPMorgan Chase Bank, N.A. [Member]
Concentration of credit risk	36.00%	29.00%	16.00%