Debt (Narrative) (Detail) (USD $)	12 Months Ended		1 Months Ended	12 Months Ended				12 Months Ended							12 Months Ended			12 Months Ended
	Dec. 31, 2012 D	Dec. 31, 2011	Jan. 16, 2008 14% Surplus Notes Due 2033 [Member]	Dec. 31, 2012 14% Surplus Notes Due 2033 [Member]		Dec. 31, 2011 14% Surplus Notes Due 2033 [Member]		Dec. 31, 2012 Investment Agreement Obligations [Member]	Dec. 31, 2011 Investment Agreement Obligations [Member]	Dec. 31, 2012 Medium-Term Notes [Member]	Dec. 31, 2011 Medium-Term Notes [Member]	Dec. 31, 2012 5.700% Senior Notes Due 2034 [Member]	Dec. 31, 2012 Cutwater-ISC [Member] Investment	Dec. 31, 2011 Cutwater-ISC [Member]	Dec. 31, 2012 Triple-A-One Funding, LLC [Member]	Dec. 31, 2012 Non Variable Interest Entities [Member] 14% Surplus Notes Due 2033 [Member]	Dec. 31, 2011 Non Variable Interest Entities [Member] 14% Surplus Notes Due 2033 [Member]	Dec. 31, 2012 Variable Interest Entity [Member]	Dec. 31, 2011 Variable Interest Entity [Member]
Debt Instrument [Line Items]
Aggregate principal amount in excess of stated amount	$ 10,000,000	$ 10,000,000
Surplus notes outstanding			1,000,000,000	940,000,000	[1]	940,000,000	[1]									940,000,000	940,000,000
Long-term debt, interest rate			14.00%	14.00%								5.70%
Long-term debt, maturity date			Jan. 15, 2033	Jan. 15, 2033
Interest rate terms			Interest rate of 14% until January 15, 2013 and thereafter have an interest rate of three-month LIBOR plus 11.26%												One-month LIBOR plus 0.75%
Debt Instrument, base reference rate			Three-month LIBOR												One-month LIBOR
Debt Instrument, basis spread on variable rate			11.26%												0.75%
Surplus notes purchased at par value	13,000,000
Surplus notes repurchased at outstanding par value	47,000,000
Surplus notes, weighted average price per unit	77.08
Repurchase of senior notes	172,000,000											172,000,000
Long-term debt, maturity year				2033								2034
Weighted average price of senior notes												100.17
Annual interest rates, minimum								0.28%	0.00%	0.00%	0.00%							0.55%	0.64%
Annual interest rates, maximum								7.38%	7.38%	8.08%	8.68%							1.71%	1.78%
Weighted average interest rate								5.04%	4.31%	2.89%	3.19%							1.28%	1.41%
Outstanding letters of credit													1,600,000
Number of pooled investment programs													2
Maximum amount of future payments under commitments														3,300,000,000
Triple-A One commercial paper maturity, days	270
Covering of the face amount of commercial paper outstanding, percentage															100.00%
Repaid value in full and borrowings under liquidity facilities															$ 360,000,000

[1] Callable on or after January 15, 2018 and every fifth anniversary thereafter at 100.00.