Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 15,864,867	$ 15,618,470
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	151,600
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	184,900
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	30,000
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	600
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	3,000
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 104,300