Net Federal Income Taxes and Cash Interest Paid (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplemental Cash Flow Information [Abstract]
Federal income taxes paid	$ 251	$ 859	$ 133
Federal income taxes recovered	170	0	19
Interest paid	$ 19,711	$ 19,387	$ 14,504