Net Federal Income Taxes and Cash Interest Paid (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes Paid Net [Abstract]
Federal income taxes paid	$ 859	$ 133	$ 195
Federal income taxes recovered	0	19	4,889
Interest paid	$ 19,387	$ 14,504	$ 8,771