Reconciliation of Income Tax Provision at Statutory Income Tax Rate and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Provision of Income Taxes [Line Items]
Statutory federal rate	35.00%	35.00%	35.00%
Dividends paid deduction	(32.50%)	(31.90%)	(31.10%)
State taxes, net of federal tax benefit	1.10%	0.90%	0.80%
Permanent differences	0.30%	0.40%	0.10%
Other items, net	(0.30%)	(0.80%)	(1.40%)
Effective income tax rate, Total	3.60%	3.60%	3.40%