Summary of Employee Stock Purchase Plan Activity and Expense (Detail) - 2013 Employee Stock Purchase Plan [Member] - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares purchased	92,086	75,094	102,698
Proceeds received (in millions)	$ 5.1	$ 2.3	$ 1.4

Number of shares issued during the period as a result of an employee stock purchase plan.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration