Income Taxes - Summary of Reconciliation of Statutory Federal Tax and Effective Income Tax Rates (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Effective Income Tax Rates Continuing Operations
Federal income tax expense (1)	[1]	$ 25,351	$ 22,759	$ 23,865
State income taxes, net of federal benefit		1,894	3,422	3,062
ESOP dividend deduction		(257)	(264)	(263)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		(14,299)	0	0
Other		1,620	2,424	241
Total Income Tax Expense		$ 14,309	$ 28,341	$ 26,905
Effective Income Tax Rate (2)	[2]	19.75%	38.81%	39.54%

[1] Federal income taxes were calculated at 35 percent for each year represented.

[2] Effective tax rate for 2017 includes the impact of the revaluation of deferred tax assets and liabilities for our unregulated businesses due to implementation of the TCJA