Reconciliation of Income Tax Provision at Statutory Income Tax Rate and Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation Of Provision Of Income Taxes [Line Items]
Statutory federal rate	21.00%	21.00%	21.00%	35.00%
Dividends paid deduction	(24.90%)	(18.90%)	(18.60%)
State taxes, net of federal tax benefit	1.90%	1.20%	1.00%
Permanent differences	2.20%	1.20%	1.00%
Charges associated with adoption of tax reform			0.60%
Deferred tax expense on Kansas lease structure	5.20%
Tax benefit of equity-based compensation	1.10%	0.10%	0.50%
Other items, net	0.80%	(0.60%)	(0.50%)
Effective income tax rate, Total	7.30%	4.00%	5.00%