Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 639,822	$ 668,440
Included in realized gains / losses	0	698
Included in other comprehensive income	41,759	(14,113)
Acquisitions	0	27,453
Sales	0	0
Amortization	3,469	3,585
Other	(15,465)	(42,862)
Transfers into (out of) Level 3	0	4,533
Ending Balance	$ 669,585	$ 647,734
Percent of total fixed maturities	4.00%	4.00%
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 12,982	$ 14,049
Included in realized gains / losses	0	0
Included in other comprehensive income	1,043	(950)
Acquisitions	0	0
Sales	0	0
Amortization	0	0
Other	(343)	(315)
Transfers into (out of) Level 3	0	0
Ending Balance	$ 13,682	$ 12,784
Percent of total fixed maturities	0.00%	0.00%
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 73,369	$ 71,581
Included in realized gains / losses	0	0
Included in other comprehensive income	5,745	8,691
Acquisitions	0	0
Sales	0	0
Amortization	3,459	3,572
Other	(4,412)	(4,928)
Transfers into (out of) Level 3	0	0
Ending Balance	$ 78,161	$ 78,916
Percent of total fixed maturities	1.00%	1.00%
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 553,471	$ 582,810
Included in realized gains / losses	0	698
Included in other comprehensive income	34,971	(21,854)
Acquisitions	0	27,453
Sales	0	0
Amortization	10	13
Other	(10,710)	(37,619)
Transfers into (out of) Level 3	0	4,533
Ending Balance	$ 577,742	$ 556,034
Percent of total fixed maturities	3.00%	3.00%