Fair Value Measurements (Summary of Changes in Fair Value of Level 3 Assets Measured on Recurring Basis) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	$ 16,538
Realized gains (losses)	(1,013)
Included in other comprehensive income (loss)	(340)
Purchases	32,557
Sales	(15,106)
Settlements	(70)
Fair value at end of period	32,566
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Realized gains (losses)	2
Included in other comprehensive income (loss)	249
Purchases	32,557
Sales	(172)
Settlements	(70)
Fair value at end of period	32,566
Non-Agency Mortgage-Backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	16,538	19,022
Realized gains (losses)	(1,015)
Net other-than-temporary impairment losses		(518)
Included in other comprehensive income (loss)	(589)	466
Sales	(14,934)
Settlements		(324)
Fair value at end of period		$ 18,646

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.

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