Federal and State Income Taxes - Summary Cash Dividends Per Common Share for Individual Shareholder (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Auction Market Preferred Securities, Stock Series [Line Items]
Cash dividends per Common Share	$ 12.46	$ 4.50
Cash Liquidating Distribution [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Cash dividends per Common Share	3.25	$ 4.50
Non-Cash Liquidating Distribution [Member]
Auction Market Preferred Securities, Stock Series [Line Items]
Cash dividends per Common Share	$ 9.21