Schedule II - Condensed Financial Information of Registrant (Parent Company Only) - Condensed Statements of Cash Flows (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Aug. 03, 2018
Cash flows from operating activities:
Net income (loss)	$ (437,000)	$ (216,000)	$ 267,000
Adjustments to reconcile net income loss to net cash used in operating activities:
Accretion (Amortization) of Discounts and Premiums, Investments	(15,000)	(63,000)	(137,000)
Net realized investment gains	(22,000)	(81,000)	(108,000)
Decrease in current income taxes payable	17,000	35,000	(35,000)
Share-based compensation	11,000	12,000	12,000
Other, net	59,000	79,000	68,000
Net cash used in operating activities	(175,000)	(311,000)	(1,543,000)
Cash flows from investing activities:
Proceeds from matured bonds	137,000	379,000	432,000
Purchases of bonds	(975,000)	(959,000)	(528,000)
Change in short-term investments	158,000	(218,000)	127,000
Proceeds from Sale and Maturity of Other Investments	374,000	81,000	159,000
Payments for (Proceeds from) Other Investing Activities	1,000	(2,000)	0
Net cash provided by investing activities	432,000	1,000,000	1,588,000
Capital Contribution to Subsidiaries	(29,000)
AMPS Proceeds	0	19,000	0
Cash flows from financing activities:
AMPS Exchange Cash Paid	0	0	(11,000)
Net Cash Provided by (Used in) Financing Activities	(303,000)	(691,000)	(585,000)
Cash and Cash Equivalents, Period Increase (Decrease)	(46,000)	(2,000)	(541,000)
Cash and cash equivalents at beginning of period	24,000	63,000
Cash and cash equivalents end of period	20,000	24,000	63,000
Cash paid during the period for:
Income taxes	11,000	21,000	35,000
Payments to Acquire Businesses, Net of Cash Acquired	(74,000)
Parent Company [Member]
Condensed Cash Flow Statements, Captions [Line Items]
AMPS Exchange Dollar Value Warrants Issued				$ 8,000
Parent Company [Member]
Cash flows from operating activities:
Net income (loss)	(437,000)	(216,000)	186,000
Adjustments to reconcile net income loss to net cash used in operating activities:
Equity in undistributed net (income) loss of non-debtor subsidiaries	423,000	223,000	(157,000)
Accretion (Amortization) of Discounts and Premiums, Investments	(6,000)	(6,000)	(7,000)
Net realized investment gains	1,000	2,000	2,000
Decrease in current income taxes payable	30,000	15,000	(15,000)
Share-based compensation	11,000	12,000	12,000
Decrease in other assets	(1,000)	(8,000)	12,000
Other, net	(10,000)	(6,000)	0
Net cash used in operating activities	11,000	16,000	32,000
Cash flows from investing activities:
Proceeds from matured bonds	46,000	86,000	230,000
Purchases of bonds	(45,000)	(2,000)	(137,000)
Change in short-term investments	89,000	(125,000)	(123,000)
Net cash provided by investing activities	16,000	(22,000)	(21,000)
Cash flows from financing activities:
AMPS Exchange Cash Paid	0
Cash and Cash Equivalents, Period Increase (Decrease)	(2,000)	(6,000)	11,000
Cash and cash equivalents at beginning of period	9,000	15,000	4,000
Cash and cash equivalents end of period	7,000	9,000	15,000
Cash paid during the period for:
Income taxes		1,000	4,000
Payments to Acquire Businesses, Net of Cash Acquired	(74,000)
Parent [Member]
Cash flows from investing activities:
Proceeds from Sale and Maturity of Other Investments	0	0	25,000
Payments for (Proceeds from) Other Investing Activities	0	0	(5,000)
AMPS Proceeds	$ 0	19,000	0
Cash flows from financing activities:
AMPS Exchange Cash Paid		$ 0	$ (11,000)