Investments - Quantitative Information about Level 3 Fair Value Measurements (Detail) $ in Thousands	Dec. 31, 2019 USD ($) number	Dec. 31, 2018 USD ($)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $	$ 18,907,147	$ 16,297,932
Collateralized debt obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $		73,369
Significant Unobservable Inputs (Level 3)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $	759,409	$ 639,822
Significant Unobservable Inputs (Level 3) | Asset- backed Securities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $	$ 13,177
Significant Unobservable Inputs (Level 3) | Asset- backed Securities | Discounted cash flows | Discount rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	0.0480
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	0.0480
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $	$ 74,104
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	0.0610
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	0.0700
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	0.0684
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $	$ 565,104
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	0.0248
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	0.0939
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	0.0368
Significant Unobservable Inputs (Level 3) | Corporates
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair Value | $	$ 107,024
Significant Unobservable Inputs (Level 3) | Corporates | Present value | Market Quotes
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0539
Significant Unobservable Inputs (Level 3) | Corporates | Present value | Market Quotes | Weighted Average
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Measurement input	1.0539