Fair Value Measurements - Quantitative Information Related to Significant Unobservable Inputs (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Mortgage-backed Securities, Privately issued [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	$ 696	$ 1,850	$ 993,247	$ 1,023,886
Valuation technique	Two independent pricing quotes	Two independent pricing quotes
Unobservable inputs/ assumptions
Mortgage-backed Securities, Privately issued [Member] | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default
Mortgage-backed Securities, Privately issued [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default
Mortgage-backed Securities, Privately issued [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default
Collateralized Debt Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	61,768	63,083	61,718	61,869
Valuation technique	Discounted cash flow	Discounted cash flow
Unobservable inputs/ assumptions	Probability of default Loss severity	Probability of default Loss severity
Collateralized Debt Obligations [Member] | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	37.00%	39.00%
Loss severity
Collateralized Debt Obligations [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	16.00%	17.00%
Loss severity
Collateralized Debt Obligations [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Probability of default	54.00%	55.00%
Loss severity	100.00%	100.00%
Other Assets and Other Liabilities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Investments	$ 12,589	$ 3,941	$ 36,119	$ 47,859
Valuation technique	Discounted cash flow	Discounted cash flow
Unobservable inputs/ assumptions	Commitment expirations	Commitment expirations
Other Assets and Other Liabilities [Member] | Weighted Average [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations weighted-average	17.00%	20.00%
Other Assets and Other Liabilities [Member] | Minimum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations	0.00%	0.00%
Other Assets and Other Liabilities [Member] | Maximum [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment expirations	90.00%	90.00%