Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 SecurityLoan	Dec. 31, 2011
Investment [Line Items]
Trading securities	$ 26.1	$ 35.0
Number of Collateralized loan obligations	4
Total Assets of the CLOs that were formed during the year	3,000.0
Total assets of partnership fund	59.7
Total liabilities of partnership fund	66.9
Total partners' capital of partnership fund	7.2
Minimum percent of aggregate assets for individual investments with fair value	5.00%
NGP Management [Member]
Investment [Line Items]
Amount of amortization and expenses associated with investment in NGP Management that reduced income from equity investments	1.1
Corporate mezzanine securities and bonds [Member]
Investment [Line Items]
Trading securities	$ 26.1	$ 35.0