Federal and State Income Taxes - Additional Information (Detail)	12 Months Ended
Dec. 31, 2017
Auction Market Preferred Securities, Stock Series [Line Items]
Percentage of taxable income distributed to shareholders	90.00%
Minimum [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Percentage of taxable income distributed to shareholders	90.00%

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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Period Type:	duration