Income Taxes - Reconciliation of Effective Tax Rate and Statutory Federal Income Tax Rate (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	9.00%	6.90%
Tax-exempt interest on municipal obligations	(1.10%)	(1.90%)
Increase in cash value of insurance policies	(0.70%)	(1.40%)
Low income housing tax credits	(1.00%)	(0.60%)
Equity compensation		(0.50%)
Nondeductible merger expenses	0.40%
Nondeductible joint beneficiary agreement expense	0.10%	0.10%
Other	0.40%	0.20%
Effective Tax Rate	28.10%	37.80%