Fair Value Measurements - Schedule of Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Restricted cash and investments	$ 39,556	$ 15,064
Fair Value Recurring Basis
Assets:
Restricted cash and investments	64,600	39,956
Marketable securities	16,899	16,957
Liabilities:
Other liabilities related to restricted investments	(8,672)
Fair Value Recurring Basis | Level 1
Assets:
Restricted cash and investments	19,775	19,481
Marketable securities	9,816	9,515
Fair Value Recurring Basis | Level 2
Assets:
Restricted cash and investments	3,050	4,467
Marketable securities	7,083	7,442
Fair Value Recurring Basis | Level 3
Assets:
Restricted cash and investments	41,775	$ 16,008
Liabilities:
Other liabilities related to restricted investments	$ (8,672)