Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 16,061,560	$ 15,618,470
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	140,800
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	203,500
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	32,000
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	500
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	3,100
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 69,000