Schedule IV - Changes in Mortgage Loans Excluding Unamortized Loan Issue Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Mortgage Loans On Real Estate [Abstract]
Balance at beginning of year	$ 1,778,264	$ 1,060,336	$ 757,500
New mortgage loans and additional advances on existing loans	50,783	717,928	612,836
Mortgage loans on real estate including additions during year	1,829,047	1,778,264	1,370,336
Collection of principal	(615,764)		(310,000)
Deductions during year	(615,764)		(310,000)
Balance at end of year	$ 1,213,283	$ 1,778,264	$ 1,060,336