Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 15,579,894	$ 14,211,745
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	138,600
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	181,700
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	22,900
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	400
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	3,500
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 165,600