Summary of Significant Accounting Policies - Additional Information (Detail)	Dec. 31, 2012
Investments in limited partnerships | Lower Limit
Significant Accounting Policies [Line Items]
Percentage of voting interest held by the company for investments accounted for under the equity method	5.00%
Investments in limited partnerships | Upper Limit
Significant Accounting Policies [Line Items]
Percentage of voting interest held by the company for investments accounted for under the equity method	50.00%
Percentage of ownership interest held by the company for investments accounted for under the cost method	5.00%
Privately Held Companies | Lower Limit
Significant Accounting Policies [Line Items]
Percentage of voting interest held by the company for investments accounted for under the equity method	20.00%
Privately Held Companies | Upper Limit
Significant Accounting Policies [Line Items]
Percentage of voting interest held by the company for investments accounted for under the equity method	50.00%
Percentage of ownership interest held by the company for investments accounted for under the cost method	20.00%