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Changes in Mortgage Loans (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mortgage Loans On Real Estate [Abstract]      
Balance at beginning of period $ 442,826 $ 437,144 $ 357,754
Additions during period:      
New mortgage loans 0 11,139 82,634
Interest 28,116 36,499 27,131
Accretion of discount 269 1,038 980
Deductions during period:      
Provision for loan loss reserve (61,000) 0 0
Collections of principal and interest (112,760) (42,994) (31,355)
Balance at close of period $ 297,451 $ 442,826 $ 437,144