Basis of Presentation of Interim Period Statements - Schedule of Reconciles of Cash, Cash Equivalents and Restricted Cash Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	$ 34,975	$ 59,835
Restricted cash, included in restricted cash and investments	70,149	60,370	$ 50,200	$ 290,600
Total cash, cash equivalents and restricted cash shown in the statement of cash flows	$ 105,124	$ 120,205