Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 14,950,006	$ 14,211,745
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	124,500
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	168,300
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	20,100
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	1,200
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	2,300
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 152,000