Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 15,618,470	$ 14,211,745
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	158,600
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	191,800
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	26,000
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	500
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	4,500
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 52,300