Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 639,822	$ 668,440	$ 623,103
Included in realized gains / losses	396	698	0
Included in other comprehensive income	32,600	(21,108)	20,964
Acquisitions	0	27,453	35,666
Sales	0	0	0
Amortization	4,609	4,753	4,931
Other	(25,042)	(44,947)	(16,224)
Transfers into Level 3	107,024	4,533	0
Transfers out of Level 3	0	0	0
Ending Balance	759,409	639,822	668,440
Change in unrealized gains (losses) for the period included in other comprehensive income for assets held at the end of the reporting period	32,600	(21,108)	20,964
Asset- backed Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	12,982	14,049	0
Included in realized gains / losses	0	0	0
Included in other comprehensive income	708	(591)	410
Acquisitions	0	0	14,000
Sales	0	0	0
Amortization	0	0	0
Other	(513)	(476)	(361)
Transfers into Level 3	0	0	0
Transfers out of Level 3	0	0	0
Ending Balance	13,177	12,982	14,049
Change in unrealized gains (losses) for the period included in other comprehensive income for assets held at the end of the reporting period	708	(591)	410
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	73,369	71,581	63,503
Included in realized gains / losses	0	0	0
Included in other comprehensive income	1,514	3,170	9,654
Acquisitions	0	0	0
Sales	0	0	0
Amortization	4,596	4,737	4,914
Other	(5,375)	(6,119)	(6,490)
Transfers into Level 3	0	0	0
Transfers out of Level 3	0	0	0
Ending Balance	74,104	73,369	71,581
Change in unrealized gains (losses) for the period included in other comprehensive income for assets held at the end of the reporting period	1,514	3,170	9,654
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	553,471	582,810	559,600
Included in realized gains / losses	396	698	0
Included in other comprehensive income	30,378	(23,687)	10,900
Acquisitions	0	27,453	21,666
Sales	0	0	0
Amortization	13	16	17
Other	(19,154)	(38,352)	(9,373)
Transfers into Level 3	107,024	4,533	0
Transfers out of Level 3	0	0	0
Ending Balance	672,128	553,471	582,810
Change in unrealized gains (losses) for the period included in other comprehensive income for assets held at the end of the reporting period	$ 30,378	$ (23,687)	$ 10,900