Investments - Quantitative Information about Level 3 Fair Value Measurements (Detail) $ in Thousands	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 21,305,287	$ 21,213,509
Collateralized debt obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value		71,598
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	705,193	$ 798,973
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 539,880
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	0.0136
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	0.0885
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	0.0305
Significant Unobservable Inputs (Level 3) | Corporates
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 101,808
Significant Unobservable Inputs (Level 3) | Corporates | Present value | Market Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0025
Significant Unobservable Inputs (Level 3) | Corporates | Present value | Market Quotes | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0025
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value	$ 63,505
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	0.0645
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	0.0645