Investments - Quantitative Information about Level 3 Fair Value Measurements (Detail) $ in Thousands	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 21,213,509	$ 18,907,147
Collateralized debt obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value		74,104
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	798,973	$ 759,409
Significant Unobservable Inputs (Level 3) | Asset- backed Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 12,870
Significant Unobservable Inputs (Level 3) | Asset- backed Securities | Discounted cash flows | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0541
Significant Unobservable Inputs (Level 3) | Asset- backed Securities | Discounted cash flows | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0541
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 71,598
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Discounted cash flows | Discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0635
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Discounted cash flows | Discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0710
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Discounted cash flows | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0698
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 612,906
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Discounted cash flows | Discount rate | Minimum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0104
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Discounted cash flows | Discount rate | Maximum
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0829
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Discounted cash flows | Discount rate | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0229
Significant Unobservable Inputs (Level 3) | Corporates
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 101,599
Significant Unobservable Inputs (Level 3) | Corporates | Present value | Market Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0005
Significant Unobservable Inputs (Level 3) | Corporates | Present value | Market Quotes | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0005