Acquisitions and Discontinued Operations - Quantitative Information about Level 3 Fair Value Measurements (Detail) (Level 3 [Member], USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Prepayment Speed Trust Loans and Portfolio Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	6.16%	10.22%	10.02%
Loss Severity [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	52.29%	52.34%	52.30%
Loan Discount Rates [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	3.12%	5.30%	4.79%
Loan Default Rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	13.84%	12.40%	13.44%
Discount Rate Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	2.43%	4.80%	4.14%
Trust Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Trust loans and portfolio loans accounted for at fair value	$ 2,468	$ 2,653	$ 2,526
Valuation Techniques	Discounted cash flow	Discounted cash flow	Discounted cash flow
Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Trust loans and portfolio loans accounted for at fair value	$ 2,118	$ 2,373	$ 2,159
Valuation Techniques	Discounted cash flow	Discounted cash flow	Discounted cash flow
Minimum [Member] | Prepayment Speed Trust Loans and Portfolio Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	4.00%	4.00%	4.00%
Minimum [Member] | Loss Severity [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	2.00%	2.00%	2.00%
Minimum [Member] | Loan Discount Rates [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	1.90%	3.00%	2.40%
Minimum [Member] | Loan Default Rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	8.13%	8.00%	8.13%
Minimum [Member] | Discount Rate Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	1.10%	2.10%	1.50%
Maximum [Member] | Prepayment Speed Trust Loans and Portfolio Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	13.50%	26.00%	26.00%
Maximum [Member] | Loss Severity [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	80.00%	80.00%	80.00%
Maximum [Member] | Loan Discount Rates [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	4.20%	8.10%	6.60%
Maximum [Member] | Loan Default Rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	22.00%	20.64%	21.50%
Maximum [Member] | Discount Rate Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	3.70%	6.90%	6.10%