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Credit Agreement - Schedule of Credit Agreements (Parenthetical) (Detail) - Credit Agreement [Member] - USD ($)
 12 Months Ended 38 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Line Of Credit Facility [Line Items]      
Outstanding letters of credit [1],[2] $ 5,070,000 $ 8,597,000 $ 5,070,000
Certain outstanding letters of credit carry fee rate originating after October 31, 2014 [3] 0.75% 0.75%  
BMO Harris Bank N.A. (“BMO Harris”) [Member]      
Line Of Credit Facility [Line Items]      
Outstanding letters of credit $ 800,000 $ 1,400,000 $ 800,000
Certain outstanding letters of credit carry fee rate originating after October 31, 2014     1.50%
BMO Harris Bank N.A. (“BMO Harris”) [Member] | Letter of Credit [Member]      
Line Of Credit Facility [Line Items]      
Letters of credit, sublimit and aggregate commitments 95,000,000 95,000,000 $ 95,000,000
BMO Harris Bank N.A. (“BMO Harris”) [Member] | Letter of Credit Sublimit [Member]      
Line Of Credit Facility [Line Items]      
Letters of credit, sublimit and aggregate commitments $ 60,000,000 $ 60,000,000 $ 60,000,000
[1] As of December 31, 2017 and 2016, outstanding letters of credit not related to the Credit Agreement totaled $0.8 million and $1.4 million, respectively, which amount is fully collateralized with restricted cash, which is in addition to the $5.1 million and $8.6 million, respectively, referenced in this Note 10.
[2] Represents letters of credit which are fully collateralized with $5.1 million and $8.6 million of restricted short-term investments as of December 31, 2017 and 2016, respectively.
[3] Certain outstanding letters of credit in the above table originating after October 31, 2014 carry a fee rate of 1.50%