Derivatives - Quantitative Information about Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Series F Agreement [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value	(537)		(826)
Discounted Cash Flow [Member] | Unobservable Inputs (Level 3) [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair value inputs, long dated treasuries	2.99%	[1]	2.82%	[1]
Fair value inputs, own credit risk			0.98%	[2]
Discounted Cash Flow [Member] | Unobservable Inputs (Level 3) [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair value inputs, long dated treasuries	3.07%	[1]	2.91%	[1]
Fair value inputs, own credit risk			1.59%	[2]

[1] Represents the forward 30 year Treasury CMT Rate.

[2] Represents credit default swap spread curve used in the valuation analysis at December 31, 2012.