Acquisitions and Discontinued Operations - Quantitative Information about Level 3 Fair Value Measurements (Detail) (Level 3 [Member], USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 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	6.04%	10.08%	9.74%
Loss Severity [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	37.67%	51.05%	49.61%
Loan Discount Rates [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	7.09%	5.04%	5.12%
Loan Default Rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	15.97%	12.60%	13.44%
Discount Rate Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	2.64%	4.30%	4.14%
Trust Loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Trust loans and portfolio loans accounted for at fair value	$ 2,283	$ 2,584	$ 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	$ 2,016	$ 2,310	$ 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	2.00%	2.60%	2.40%
Minimum [Member] | Loan Default Rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	8.02%	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%	1.80%	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	79.50%	80.00%	80.00%
Maximum [Member] | Loan Discount Rates [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	10.50%	10.50%	10.50%
Maximum [Member] | Loan Default Rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	23.78%	21.50%	21.50%
Maximum [Member] | Discount Rate Trust Securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	3.80%	6.50%	6.10%