Schedule IV - Changes in Mortgage Loans Excluding Unamortized Loan Issue Costs (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 year	$ 1,060,336	$ 757,500	$ 397,500
New mortgage loans and additional advances on existing loans	717,928	612,836	380,000
Mortgage loans on real estate including additions during year	1,778,264	1,370,336	777,500
Collection of principal		(310,000)	(20,000)
Deductions during year		(310,000)	(20,000)
Balance at end of year	$ 1,778,264	$ 1,060,336	$ 757,500