Derivatives - Quantitative Information About Level 3 Fair Value Measurements (Detail) (USD $) In Thousands, unless otherwise specified			6 Months Ended		12 Months Ended
	Jun. 30, 2013 Series F Agreement [Member]	Dec. 31, 2012 Series F Agreement [Member]	Jun. 30, 2013 Discounted Cash Flow [Member] Unobservable Inputs (Level 3) Derivative Financial Instruments, Liabilities [Member]		Dec. 31, 2012 Discounted Cash Flow [Member] Unobservable Inputs (Level 3) Derivative Financial Instruments, Liabilities [Member] Minimum		Dec. 31, 2012 Discounted Cash Flow [Member] Unobservable Inputs (Level 3) Derivative Financial Instruments, Liabilities [Member] Maximum
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair value inputs, long dated treasuries			3.54%	[1]	2.82%	[1]	2.91%	[1]
Fair value inputs, own credit risk					0.98%	[2]	1.59%	[2]
Fair Value	$ (214)	$ (826)

[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.