Investments (Net Investment Income) - Additional Information (Detail) $ in Thousands	Dec. 31, 2018 USD ($) Bank Accounts	Dec. 31, 2017 USD ($) Bank Accounts
Schedule of Investments [Line Items]
Cash and cash equivalents	$ 239,458	$ 255,884
Deposits representing cash and cash equivalents in percentage	75.40%	34.10%
Number of national banks | Bank	3	3
Number of custodial accounts | Accounts	2	3
Federal Deposit Insurance Corporation's coverage limit for insured deposit accounts	$ 250	$ 250
Custodial Accounts [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents	73,511	38,543
Bank Deposits [Member]
Schedule of Investments [Line Items]
Cash and cash equivalents	$ 180,508	$ 87,092