Reinsurance - Current Ratings from Rating Agencies and Unsecured Net Amounts Due from Reinsurers Whose Aggregate Balance Exceeded 3% of Stockholders' Equity (Detail) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	$ 655,119	$ 773,345
Florida Hurricane Catastrophe Fund
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	92,193	134,411
D E Shaw Re (Bermuda) Ltd
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	27,399	16,680
Cosaint Re Pte. Ltd.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	17,467	0
Upsilon RFO Re Ltd
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	0	11,201
Lumen Re Ltd.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	0	8,913
AM Best Company A Plus Rating | Moody's, Aa3 Rating | Standard & Poor's, AA- Rating | Allianz Risk Transfer
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	243,564	285,323
AM Best Company A Plus Rating | Moody's, Aa3 Rating | Standard & Poor's, AA- Rating | Munich Reinsurance America Inc.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	12,156	14,616
AM Best Company A Plus Rating | Moody's A1 Rating | Standard & Poor's, A Plus Rating | Renaissance Reinsurance Ltd.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	30,896	38,768
AM Best Company A Plus Rating | Moody's A1 Rating | Standard & Poor's, A Plus Rating | Everest Reinsurance Co
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	12,471	11,536
AM Best, A Rating | Standard & Poor's, A Plus Rating | Various Lloyd’s of London Syndicates
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	86,774	101,482
AM Best, A Rating | Moody's, A2 Rating | Standard & Poor's, A Rating | Markel Bermuda Ltd.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	44,361	50,981
AM Best, A Rating | Moody's, A3 Rating | Standard & Poor's, A Plus Rating | DaVinci Reinsurance Ltd.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	41,097	48,115
AM Best, A++ Rating | Moody's, Aa3 Rating | Standard & Poor's, AA Rating | Chubb Tempest Reinsurance, Ltd.
Ceded Credit Risk [Line Items]
Reinsurance unsecured amounts due from reinsurers	$ 46,741	$ 51,319