Fair Value Measurements - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	$ 1,007	$ 1,106	$ 1,064
Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	26	52	25
Level 3 [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Other investments	168	202	181
Level 3 [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Impaired loans	26	52	25
Goodwill	$ 979	$ 979	$ 979
Level 3 [Member] | EBITDA Multiple [Member] | Recurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	(6.00%)	(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	5.80%	4.90%	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	7.20%	6.00%	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	(6.30%)	(6.10%)	(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	4.70%	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	9.60%	10.80%	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	(4.10%)	(2.80%)	(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	1.00%	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	4.80%	4.40%	5.70%
Level 3 [Member] | Discount [Member] | Nonrecurring [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Percentage of significant unobservable input weighted-average	(35.00%)	(35.00%)	(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	(10.00%)	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