Equity - Noncontrolling Interests (Detail) - Non-controlling Interests [Member] - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity [Line Items]
Preferred stock, shares outstanding	125	125
Preferred stock, dividend rate (as a percent)	12.50%	12.50%
Common Class B [Member]
Equity [Line Items]
Total common equity (as a percent)	43.00%	44.00%