SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Cash Flows (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents
Cash and cash equivalents	$ 191,218	$ 118,756	$ 136,988	$ 113,354
Restricted cash	6,677	9,861	5,306	13,854
Pledged securities, at fair value	97,859	84,850	72,190	67,719
Total cash, cash equivalents, restricted cash, and restricted cash equivalents	$ 295,754	$ 213,467	$ 214,484	$ 194,927