Schedule IV - Changes in Mortgage Loans Excluding Unamortized Loan Issue Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Mortgage Loans on Real Estate [Abstract]
Balance at beginning of year	$ 757,500	$ 397,500	$ 388,650
New mortgage loans and additional advances on existing loans	612,836	380,000	12,500
Mortgage loans on real estate including additions during year	1,370,336	777,500	401,150
Collection of principal	(310,000)	(20,000)	(3,650)
Deductions during year	(310,000)	(20,000)	(3,650)
Balance at end of year	$ 1,060,336	$ 757,500	$ 397,500