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Schedule of Credit Quality of Commercial Mortgage Loans Held-For-Investment (Detail) - Commercial Mortgage Loans - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment $ 1,043,187 $ 1,455,156
Estimated Fair Value $ 1,044,531 $ 1,465,113
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 $ 954,348 $ 1,381,742
Debt Service Coverage Ratio 1.00x - 1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment 88,839 62,060
Debt Service Coverage Ratio Less than 1.00x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment   11,354
Loan To Value Ratio Less Than 65%    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment 780,472 833,246
Estimated Fair Value $ 782,640 $ 848,702
Recorded Investment, % of Total 74.80% 57.30%
Estimated Fair Value, % of Total 74.90% 57.90%
Loan To Value Ratio Less Than 65% | Debt Service Coverage Ratio >1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment $ 731,577 $ 785,149
Loan To Value Ratio Less Than 65% | Debt Service Coverage Ratio 1.00x - 1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment 48,895 36,743
Loan To Value Ratio Less Than 65% | Debt Service Coverage Ratio Less than 1.00x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment   11,354
Loan To Value Ratio 65% to 75%    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment 262,715 606,505
Estimated Fair Value $ 261,891 $ 601,587
Recorded Investment, % of Total 25.20% 41.70%
Estimated Fair Value, % of Total 25.10% 41.10%
Loan To Value Ratio 65% to 75% | Debt Service Coverage Ratio >1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment $ 222,771 $ 581,188
Loan To Value Ratio 65% to 75% | Debt Service Coverage Ratio 1.00x - 1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment $ 39,944 25,317
Loan To Value Ratio 76% to 80%    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment   10,493
Estimated Fair Value   $ 10,494
Recorded Investment, % of Total 0.00% 0.70%
Estimated Fair Value, % of Total 0.00% 0.70%
Loan To Value Ratio 76% to 80% | Debt Service Coverage Ratio >1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment   $ 10,493
Loan To Value Ratio Greater than 80%    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment   4,912
Estimated Fair Value   $ 4,330
Recorded Investment, % of Total 0.00% 0.30%
Estimated Fair Value, % of Total 0.00% 0.30%
Loan To Value Ratio Greater than 80% | Debt Service Coverage Ratio >1.20x    
Financing Receivable, Recorded Investment [Line Items]    
Recorded Investment   $ 4,912