Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 759,409	$ 639,822
Included in realized gains / losses	1,213	0
Included in other comprehensive income	(39,823)	15,137
Acquisitions	0	0
Sales	0	0
Amortization	1,141	1,165
Other	(22,405)	(3,401)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 699,535	$ 652,723
Percent of total fixed maturities	3.90%	3.80%
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 13,177	$ 12,982
Included in realized gains / losses	0	0
Included in other comprehensive income	(276)	298
Acquisitions	0	0
Sales	0	0
Amortization	0	0
Other	(134)	(126)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 12,767	$ 13,154
Percent of total fixed maturities	0.00%	0.10%
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 74,104	$ 73,369
Included in realized gains / losses	0	0
Included in other comprehensive income	(5,475)	4,176
Acquisitions	0	0
Sales	0	0
Amortization	1,139	1,162
Other	(1,395)	(1,458)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 68,373	$ 77,249
Percent of total fixed maturities	0.40%	0.40%
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 672,128	$ 553,471
Included in realized gains / losses	1,213	0
Included in other comprehensive income	(34,072)	10,663
Acquisitions	0	0
Sales	0	0
Amortization	2	3
Other	(20,876)	(1,817)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending Balance	$ 618,395	$ 562,320
Percent of total fixed maturities	3.50%	3.30%