Schedule of Credit Quality of Commercial Mortgage Loans Held-For-Investment (Detail) - Commercial Mortgage Loans - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	$ 853,073	$ 1,043,187
Estimated Fair Value	$ 852,034	$ 1,044,531
Recorded Investment, % of Total	100.00%	100.00%
Estimated Fair Value, % of Total	100.00%	100.00%
Debt Service Coverage Ratio >1.20x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	$ 678,279	$ 954,348
Debt Service Coverage Ratio 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	137,348	88,839
Debt Service Coverage Ratio Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	37,446
Loan To Value Ratio Less Than 65%
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	754,129	780,472
Estimated Fair Value	$ 752,667	$ 782,640
Recorded Investment, % of Total	88.40%	74.80%
Estimated Fair Value, % of Total	88.30%	74.90%
Loan To Value Ratio Less Than 65% \| Debt Service Coverage Ratio >1.20x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	$ 628,279	$ 731,577
Loan To Value Ratio Less Than 65% \| Debt Service Coverage Ratio 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	88,404	48,895
Loan To Value Ratio Less Than 65% \| Debt Service Coverage Ratio Less than 1.00x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	37,446
Loan To Value Ratio 65% to 75%
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	98,944	262,715
Estimated Fair Value	$ 99,367	$ 261,891
Recorded Investment, % of Total	11.60%	25.20%
Estimated Fair Value, % of Total	11.70%	25.10%
Loan To Value Ratio 65% to 75% \| Debt Service Coverage Ratio >1.20x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	$ 50,000	$ 222,771
Loan To Value Ratio 65% to 75% \| Debt Service Coverage Ratio 1.00x - 1.20x
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment	$ 48,944	$ 39,944
Loan To Value Ratio 76% to 80%
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment, % of Total	0.00%	0.00%
Estimated Fair Value, % of Total	0.00%	0.00%
Loan To Value Ratio Greater than 80%
Financing Receivable, Recorded Investment [Line Items]
Recorded Investment, % of Total	0.00%	0.00%
Estimated Fair Value, % of Total	0.00%	0.00%

Percentage of Financing Receivable Recorded Investment

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- Definition

Percentage Of Loans Receivable Fair Value

+ References

No definition available.

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- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

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