Summary of Quantitative Inputs and Assumptions of Level 3 Assets and Liabilities (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended								12 Months Ended													12 Months Ended		9 Months Ended		12 Months Ended		9 Months Ended				12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2012 Structured securities	Dec. 31, 2012 Structured securities Residential mortgage-backed securities	Sep. 30, 2013 Limited partnership	Dec. 31, 2012 Limited partnership	Sep. 30, 2013 Fixed maturity AFS securities	Sep. 30, 2013 Fixed maturity AFS securities Asset-backed securities	Sep. 30, 2013 Fixed maturity AFS securities Corporate securities	Dec. 31, 2012 GMWB		Sep. 30, 2013 GMWB		Dec. 31, 2011 GMWB		Sep. 30, 2013 GMIB		Dec. 31, 2012 GMIB		Dec. 31, 2011 GMIB		Sep. 30, 2013 Stand Alone Living Benefit	Dec. 31, 2012 Level 3 Structured securities	Dec. 31, 2012 Level 3 Structured securities Weighted Average	Sep. 30, 2013 Level 3 Limited partnership		Dec. 31, 2012 Level 3 Limited partnership		Sep. 30, 2013 Level 3 Fixed maturity AFS securities Asset-backed securities	Sep. 30, 2013 Level 3 Fixed maturity AFS securities Corporate securities	Sep. 30, 2013 Level 3 GMWB	Sep. 30, 2013 Level 3 GMWB Weighted Average	Dec. 31, 2012 Level 3 GMWB Weighted Average	Sep. 30, 2013 Level 3 GMIB	Dec. 31, 2012 Level 3 GMIB	Sep. 30, 2013 Level 3 GMIB Weighted Average	Dec. 31, 2012 Level 3 GMIB Weighted Average	Sep. 30, 2013 Level 3 Stand Alone Living Benefit
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]
Fair Value	$ 21,416	$ 6,639	$ 91	$ 91	$ 4,802	$ 6,548	$ 16,614	$ 14,813	$ 1,801
Future policy benefits (embedded derivatives), fair value	(34,398)	(17,397)								74,022	[1]	28,776	[1]	108,637	[1]							41
Valuation Technique										Discounted cash flow													Discounted cash flows		Not applicable	[2]	Not applicable	[2]	Broker & recent purchase price	Broker	Discounted cash flow			Discounted cash flow	Discounted cash flow			based on fees earned	[3]
Constant prepayment rate																								1.00%
Future policy benefits (embedded derivatives) - Reinsurance, fair value																$ (63,215)	[1]	$ (91,419)	[1]	$ (94,517)	[1]
Probability of default																								12.00%
Loss severity																								65.00%
Own credit risk	0.55%	0.80%																														0.55%	0.80%			0.55%	0.80%
Long-term volatility	25.00%	25.00%																														25.00%	25.00%			25.00%	25.00%

[1] Recorded as a component of future policy benefits in the Balance Sheets.

[2] The Company has an investment in a limited partnership for which the fair value is derived from management's review of the underlying financial statements that were prepared on a GAAP basis. Management did not make any adjustments to the valuation from the underlying financial statements. As a result, inputs are not developed by management to determine the fair value measurement for this investment.

[3] The SALB is a relatively new product with fewer than 100 policies. Due to the small size of this block the liability was determined based on fees earned.