Reconciliation of Income Tax Provision at Statutory Income Tax Rate and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Provision of Income Taxes [Line Items]
Statutory federal rate	35.00%	35.00%	35.00%
Dividends paid deduction	(31.90%)	(31.10%)	(30.70%)
State taxes, net of federal tax benefit	0.90%	0.80%	1.10%
Permanent differences	0.40%	0.10%	3.00%
Impact of REIT election			(87.00%)
Other items, net	(0.80%)	(1.40%)	(1.00%)
Effective income tax rate, Total	3.60%	3.40%	(79.60%)