Acquisitions and Discontinued Operations - Significant Unobservable Inputs Used to Measure Fair Value (Detail) (Level 3 [Member], USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Trust Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Valuation Techniques	Discounted cash flow
Trust loans and portfolio loans accounted for at fair value	$ 2,584
Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Valuation Techniques	Discounted cash flow
Trust loans and portfolio loans accounted for at fair value	$ 2,310
Prepayment Speed Trust Loan [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	10.22%
Prepayment Speed Trust Loan [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	4.00%
Prepayment Speed Trust Loan [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	26.00%
Expected Credit Losses Trust Loan [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	52.34%
Expected Credit Losses Trust Loan [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	2.00%
Expected Credit Losses Trust Loan [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	80.00%
Discount Rate Trust Loan [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	4.90%
Discount Rate Trust Loan [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	2.60%
Discount Rate Trust Loan [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	7.30%
Expected Defaults Trust Loan [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	12.60%
Expected Defaults Trust Loan [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	8.00%
Expected Defaults Trust Loan [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	21.50%
Discount Rate Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	4.30%
Discount Rate Trust Securities [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	1.80%
Discount Rate Trust Securities [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	6.50%