Acquisitions and Discontinued Operations - Significant Unobservable Inputs Used to Measure Fair Value (Detail) (Level 3 [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 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%	10.02%	10.22%
Loss severity [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	52.29%	52.30%	52.34%
Loan discount rates [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	3.89%	4.79%	4.70%
Loan default rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	13.84%	13.44%	18.58%
Discount rate trust securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	3.29%	4.14%	4.10%
Trust loans [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Valuation Techniques	Discounted cash flow	Discounted cash flow	Discounted cash flow
Trust loans and portfolio loans accounted for at fair value	$ 2,487	$ 2,526	$ 2,788
Trust securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Valuation Techniques	Discounted cash flow	Discounted cash flow	Discounted cash flow
Trust loans and portfolio loans accounted for at fair value	$ 2,126	$ 2,159	$ 2,512
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	1.80%	2.40%	2.50%
Minimum [Member] | Loan default rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	8.13%	8.13%	3.75%
Minimum [Member] | Discount rate trust securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	1.10%	1.50%	1.80%
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	5.50%	6.60%	8.40%
Maximum [Member] | Loan default rate [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	22.00%	21.50%	40.00%
Maximum [Member] | Discount rate trust securities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	5.00%	6.10%	7.20%