Investments - Quantitative Information about Level 3 Fair Value Measurements (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 16,969,325	$ 15,245,861
Collateralized debt obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	71,581	63,503
Significant Unobservable Inputs (Level 3)
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	668,440	623,103
Significant Unobservable Inputs (Level 3) | Asset-backed securities | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 14,049
Discount rate	5.35%
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 71,581	$ 63,503
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	71,581
Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 582,810
Minimum | Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	7.00%
Minimum | Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	2.97%
Maximum | Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	8.25%
Maximum | Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	7.27%
Weighted Average | Significant Unobservable Inputs (Level 3) | Asset-backed securities | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	5.35%
Weighted Average | Significant Unobservable Inputs (Level 3) | Collateralized debt obligations | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	8.03%
Weighted Average | Significant Unobservable Inputs (Level 3) | Private placement fixed maturities | Determination of credit spread
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate	3.93%