Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 14,945,479	$ 15,618,470
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	142,500
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	191,600
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	25,900
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	600
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	2,800
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 103,000