COMMITMENTS AND CONTINGENCIES - Administrative and Marketing Office Space and Building (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) city	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2013 USD ($)
Future minimum rental payments required under operating leases
Outstanding mortgage loan commitments	$ 685,300	$ 572,300
Average rate (as a percent)	4.42%	4.14%
Administrative and marketing office space
Operating leased assets
Number of cities in which the company leases properties | city	17
Rental expense	$ 8,300	$ 7,800	$ 6,700
Aggregate annualized rent	8,300
Future minimum rental payments required under operating leases
2019	5,454
2020	3,707
2021	3,393
2022	3,129
2023	3,171
Thereafter	$ 5,418
Administrative and marketing office space | Minimum
Operating leased assets
Lease period (in years)	1 year
Administrative and marketing office space | Maximum
Operating leased assets
Lease period (in years)	9 years
Building contiguous to home office
Future minimum rental payments required under operating leases
Approximate price for which the company may purchase building at the end of lease term				$ 75,000