Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Escrow cash	$ 604,100
Fair Value Recurring Basis
Assets:
Restricted cash and investments	11,528	$ 13,153
Marketable securities	17,057	17,631
Escrow cash	604,100
Fair Value Recurring Basis | Level 1
Assets:
Restricted cash and investments	7,600	9,055
Marketable securities	9,486	7,906
Escrow cash	604,100
Fair Value Recurring Basis | Level 2
Assets:
Restricted cash and investments	3,928	4,098
Marketable securities	$ 7,571	$ 9,725