Subsequent Events - Additional Information (Detail) (USD $) In Billions, unless otherwise specified	12 Months Ended	1 Months Ended
	Dec. 31, 2014	Jan. 31, 2015	Feb. 28, 2015
Subsequent Event [Line Items]
Interest rate on the borrowings on the credit facility description	1.25% above the London Interbank Offered Rate.
Credit facility fee (per annum)	0.25%
Maturity of revolving credit facilities	2016-04
New syndicated revolving credit facility maturing in April 2016	$ 4
Subsequent Event
Subsequent Event [Line Items]
New syndicated revolving credit facility		$ 4
Interest rate on the borrowings on the credit facility description		1.075% above the London Interbank Offered Rate
Credit facility fee (per annum)		0.175%
Maturity of revolving credit facilities		2020-01
Subsequent Event | HETCO [Member]
Subsequent Event [Line Items]
Interest sold in energy trading joint venture, HETCO			In February 2015, the Corporation sold its interest in the energy trading joint venture, HETCO, which was renamed Hartree Partners, LP. Pursuant to the terms of the sale, Hartree is permitted to continue to utilize the Corporation’s guarantees issued in favor of Hartree's existing counterparties until November 12, 2015, provided that new trades are for a period of one year or less, comply with certain credit requirements, and net exposures remain within VAR limits previously applied by the Corporation.
Maximum guarantee period on trading joint venture, HETCO			Nov. 12, 2015