Credit Agreement - Schedule of Credit Agreements (Detail) - Credit Agreement [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	[1]
Credit Agreements:
Credit facility remaining availability		$ 47,782	$ 89,930
Outstanding letters of credit	[2],[3]	2,218	5,070
Availability of additional letters of credit	[4]	$ 22,782	$ 54,930
Weighted average annual interest rate		0.00%	0.00%
Commitment fee rate		0.30%	0.25%
Letter of credit fee rate		1.25%	0.75%

[1] Amounts listed as of and for the year ended December 31, 2017 relate to our previous credit agreement.

[2] As of December 31, 2017, $5.1 million of restricted short-term investments provided collateral for our letters of credit.

[3] As of December 31, 2018 and 2017, outstanding letters of credit not related to the credit agreements totaled $0.3 million and $0.8 million, respectively, which amounts are fully collateralized with restricted cash, which is in addition to the $5.1 million referenced in this Note 11 as of December 31, 2017.

[4] The letters of credit sublimit of $25.0 million and $60.0 million as of December 31, 2018 and 2017, respectively, under the credit agreements is part of, not in addition to, the $50.0 million and $95.0 million aggregate commitments as of December 31, 2018 and 2017, respectively.