Fair Value Measurements - Quantitative Information Related to Significant Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Privately Issued Mortgage-Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	$ 3,816	$ 1,023,886
Valuation technique	Two independent price quotes	Discounted cash flow
Unobservable inputs/assumptions	-	Probability of default and Loss severity
Privately Issued Mortgage-Backed Securities [Member] | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default		19.00%
Loss severity		51.00%
Privately Issued Mortgage-Backed Securities [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default		1.00%
Loss severity		32.00%
Privately Issued Mortgage-Backed Securities [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default		40.00%
Loss severity		82.00%
Collateralized Debt Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	59,303	61,869
Valuation technique	Discounted cash flow	Discounted cash flow
Unobservable inputs/assumptions	Probability of default and Loss severity	Probability of default and Loss severity
Collateralized Debt Obligations [Member] | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	39.00%	47.00%
Collateralized Debt Obligations [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	13.00%	22.00%
Loss severity
Collateralized Debt Obligations [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	55.00%	69.00%
Loss severity	100.00%	100.00%
Net Other Assets (Liabilities) [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	$ 13,075	$ 47,859
Valuation technique	Discounted cash flow	Discounted cash flow
Unobservable inputs/assumptions	Commitment expirations	Commitment expirations
Net Other Assets (Liabilities) [Member] | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations weighted-average	21.00%	20.00%
Net Other Assets (Liabilities) [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations	0.00%	0.00%
Net Other Assets (Liabilities) [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations	91.00%	69.00%