Federal Income Taxes - Additional Information (Detail)	12 Months Ended			36 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Income Tax Disclosure [Abstract]
Tax Adjustments, Settlements, and Unusual Provisions	$ 1,600,000
Cumulative Pre-Tax Income				3 years
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Extraordinary Items, Noncontrolling Interest	19,200,000	$ 67,300,000	$ 26,800,000	$ 98,700,000
consecutive years of pre-tax income				3 years
Percentage Ceded To Reinsurers					75.00%
Specialty Insurance Segment, former RED Program Business, Number of Programs		1
Income Tax Benefit, Deferred		$ 82,600,000
Deferred tax asset related to net operating loss carryforwards	58,900,000	41,800,000		$ 41,800,000	$ 41,800,000
Balance for uncertain tax positions	0
Interest and penalties payable	$ 0	$ 0		$ 0	$ 0