Debt - Covenants - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015
Debt Instrument [Line Items]
Percentage of dividends which could be paid from adjusted operating funds	95.00%
Percentage of dividends which could be paid from operation funds	95.00%
Maximum percentage of total unencumbered assets	150.00%
Leverage ratio	70.00%
Unsecured Notes [Member]
Debt Instrument [Line Items]
Leverage ratio	77.50%
Two Thousand Sixteen June Unsecured Notes [Member]
Debt Instrument [Line Items]
Leverage ratio	60.00%
Two Thousand Sixteen September Unsecured Notes [Member]
Debt Instrument [Line Items]
Leverage ratio	65.00%