Credit Agreement - Schedule of Credit Agreements (Parenthetical) (Detail) - Credit Agreement [Member] - USD ($)		Dec. 31, 2016	Dec. 31, 2015
Line Of Credit Facility [Line Items]
Outstanding letters of credit	[1],[2]	$ 8,597,000	$ 10,446,000
BMO Harris Bank N.A. (“BMO Harris”) [Member]
Line Of Credit Facility [Line Items]
Letter of credit collateralized of restricted cash		8,600,000	10,400,000
Outstanding letters of credit		1,400,000	1,400,000
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
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

[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.