Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Outstanding Value Of Letters Of Credit	$ 201.5	$ 331.0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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