Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	18	101	25
Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	191	216	181
Level 3 [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	18	101	25
Goodwill	979	917	979
Level 3 [Member] | EBITDA multiple [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	5.90%	5.90%	5.90%
Level 3 [Member] | EBITDA multiple [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	5.50%	5.30%	5.50%
Level 3 [Member] | EBITDA multiple [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	6.00%	6.50%	6.00%
Level 3 [Member] | EBITDA multiple (where applicable) [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	5.80%	6.37%	6.10%
Level 3 [Member] | EBITDA multiple (where applicable) [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	5.00%	5.50%	5.00%
Level 3 [Member] | EBITDA multiple (where applicable) [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	6.00%	12.00%	8.50%
Level 3 [Member] | Revenue multiple (where applicable) [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	4.05%	2.93%	4.80%
Level 3 [Member] | Revenue multiple (where applicable) [Member] | Recurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	0.30%	0.20%	0.30%
Level 3 [Member] | Revenue multiple (where applicable) [Member] | Recurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	5.30%	4.70%	5.70%
Level 3 [Member] | Discount [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	38.00%	31.00%	45.00%
Level 3 [Member] | Discount [Member] | Nonrecurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	0.00%	0.00%	0.00%
Level 3 [Member] | Discount [Member] | Nonrecurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	100.00%	100.00%	100.00%
Level 3 [Member] | Earnings multiple of peers [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Significant unobservable input weighted-average	11.25	10.01	11.25
Level 3 [Member] | Earnings multiple of peers [Member] | Nonrecurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Significant unobservable input weighted-average	9.70	8.30	9.70
Level 3 [Member] | Earnings multiple of peers [Member] | Nonrecurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Significant unobservable input weighted-average	14.20	11.90	14.20
Level 3 [Member] | Equity multiple of peers [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Significant unobservable input weighted-average	1.09	1.27	1.09
Level 3 [Member] | Equity multiple of peers [Member] | Nonrecurring [Member] | Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Significant unobservable input weighted-average	0.95	1.21	0.95
Level 3 [Member] | Equity multiple of peers [Member] | Nonrecurring [Member] | Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Significant unobservable input weighted-average	1.17	1.32	1.17
Level 3 [Member] | Control premium [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	30.00%	32.00%	30.00%
Level 3 [Member] | Weighted-average cost of capital [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	13.00%	15.00%	13.00%
Level 3 [Member] | Other Investments [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value measurement, valuation technique	Individual analysis of the condition of each investment	Individual analysis of the condition of each investment	Individual analysis of the condition of each investment
Level 3 [Member] | Impaired Loans [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value measurement, valuation technique	Fair value of underlying collateral	Fair value of underlying collateral	Fair value of underlying collateral
Level 3 [Member] | Goodwill [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value measurement, valuation technique	Discounted cash flow and market data	Discounted cash flow and market data	Discounted cash flow and market data