Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 576,779	$ 359,281	$ 286,363
Included in realized gains/losses	1,182	15,925	0
Included in other comprehensive income	(523)	31,244	(6,697)
Acquisitions	38,600	186,366	129,755
Sales		(16,050)	0
Amortization	5,553	5,532	2,786
Other	(19,533)	(5,519)	(2,121)
Transfers into (out of) Level 3	0	0	(50,805)
Ending Balance	602,058	576,779	359,281
Asset-backed Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	0	0	7,981
Included in realized gains/losses	0	0	0
Included in other comprehensive income	0	0	426
Acquisitions	0	0	0
Sales		0	0
Amortization	0	0	(57)
Other	0	0	0
Transfers into (out of) Level 3	0	0	(8,350)
Ending Balance	0	0	0
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	63,232	58,205	46,571
Included in realized gains/losses	0	15,924	0
Included in other comprehensive income	11,365	3,323	10,083
Acquisitions	0	0	0
Sales		(16,049)	0
Amortization	5,536	5,519	2,838
Other	(9,751)	(3,690)	(1,287)
Transfers into (out of) Level 3	0	0	0
Ending Balance	70,382	63,232	58,205
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	512,714	300,300	231,072
Included in realized gains/losses	1,182	1	0
Included in other comprehensive income	(11,925)	27,864	(17,243)
Acquisitions	38,600	186,366	129,755
Sales		(1)	0
Amortization	17	13	5
Other	(9,782)	(1,829)	(834)
Transfers into (out of) Level 3	0	0	(42,455)
Ending Balance	530,806	512,714	300,300
Equities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	833	776	739
Included in realized gains/losses	0	0	0
Included in other comprehensive income	37	57	37
Acquisitions	0	0	0
Sales		0	0
Amortization	0	0	0
Other	0	0	0
Transfers into (out of) Level 3	0	0	0
Ending Balance	$ 870	$ 833	$ 776

Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset, amortization.

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- Definition

Other adjustments, including capitalized interest and foreign exchange.

+ References

No definition available.

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

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
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

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
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in other comprehensive income, arising from assets, liabilities and financial instruments classified in shareholders' equity measured at fair value on a recurring basis using unobservable inputs (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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