Credit Agreement - Schedule of Credit Agreements (Detail) - Credit Agreement [Member] - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Credit Agreement:
Credit facility remaining availability		$ 86,403	$ 46,554
Credit facility borrowings			38,000
Outstanding letters of credit	[1],[2]	8,597	10,446
Availability of additional letters of credit	[3]	51,403	49,554
Weighted average daily revolving credit borrowings for the year ended		$ 19	$ 3
Weighted average annual interest rate		1.93%	1.92%
Commitment fee rate		0.25%	0.25%
Letter of credit fee rate		0.75%	0.75%

[1] As of December 31, 2016 and 2015, outstanding letters of credit not related to the Credit Agreement totaled $1.4 million for each year, respectively, which amount is fully collateralized with restricted cash and investments, which is in addition to the $8.6 million and $10.4 million, respectively, referenced in this Note 10.

[2] Represents letters of credit which are fully collateralized with $8.6 million and $10.4 million of restricted cash as of December 31, 2016 and 2015, respectively.

[3] The letters of credit sublimit of $60.0 million under the Credit Agreement is part of, not in addition to, the $95.0 million aggregate commitments.