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, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	$ 623,103	$ 601,188	$ 575,946
Included in realized gains/losses	0	788	1,182
Included in other comprehensive income	20,964	2,460	(560)
Acquisitions	35,666	33,662	38,600
Sales	0	0	0
Amortization	4,931	5,203	5,553
Other	(16,224)	(20,198)	(19,533)
Transfers into (out of) Level 3(2)	0	0	0
Ending Balance	668,440	623,103	601,188
Asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	0	0	0
Included in realized gains/losses	0	0	0
Included in other comprehensive income	410	0	0
Acquisitions	14,000	0	0
Sales	0	0	0
Amortization	0	0	0
Other	(361)	0	0
Transfers into (out of) Level 3(2)	0	0	0
Ending Balance	14,049	0	0
Collateralized debt obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	63,503	70,382	63,232
Included in realized gains/losses	0	0	0
Included in other comprehensive income	9,654	(3,943)	11,365
Acquisitions	0	0	0
Sales	0	0	0
Amortization	4,914	5,186	5,536
Other	(6,490)	(8,122)	(9,751)
Transfers into (out of) Level 3(2)	0	0	0
Ending Balance	71,581	63,503	70,382
Corporates
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation
Beginning Balance	559,600	530,806	512,714
Included in realized gains/losses	0	788	1,182
Included in other comprehensive income	10,900	6,403	(11,925)
Acquisitions	21,666	33,662	38,600
Sales	0	0	0
Amortization	17	17	17
Other	(9,373)	(12,076)	(9,782)
Transfers into (out of) Level 3(2)	0	0	0
Ending Balance	$ 582,810	$ 559,600	$ 530,806