Reconciliation of Income Tax Provision at Statutory Income Tax Rate and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Provision Of Income Taxes [Line Items]
Statutory federal rate	35.00%	35.00%	35.00%
Dividends paid deduction	(31.30%)	(32.50%)	(31.90%)
State taxes, net of federal tax benefit	1.20%	1.10%	0.90%
Permanent differences	0.60%	0.30%	0.40%
Charges associated with adoption of tax reform	2.40%
Tax benefit of equity-based compensation	(0.50%)
Other items, net	(0.20%)	(0.30%)	(0.80%)
Effective income tax rate, Total	7.20%	3.60%	3.60%