Net Federal Income Taxes and Cash Interest Paid (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flow Supplemental Disclosures [Line Items]
Federal income taxes paid	$ 195	$ 104	$ 13,997
Federal income taxes recovered	4,889	2	136
Interest paid	$ 8,771	$ 3,926	$ 804