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