Federal Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	35.00%	35.00%	35.00%
Corporate income tax rate due to enactment of TCJA	21.00%
Document Period End Date	Dec. 31, 2017
Impact of TCJA at enactment	$ 36,400,000	$ 0	$ 0
Pre-tax provisional change in discounted loss reserves attributable to TCJA	50,900,000
Tax Adjustments, Settlements, and Unusual Provisions	1,600,000
Deferred tax asset related to net operating loss carryforwards	34,300,000	58,900,000
Balance for uncertain tax positions	0
Interest and penalties payable	$ 0	$ 0