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, 2016	Mar. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 602,058	$ 576,779
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	5,970	22,182
Acquisitions	15,800	8,000
Sales	0	0
Amortization	1,338	1,414
Other	(4,003)	(4,975)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 621,163	$ 603,400
Percent of total fixed maturity and equity securities	4.30%	4.10%
Collateralized debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 70,382	$ 63,232
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	(3,598)	13,556
Acquisitions	0	0
Sales	0	0
Amortization	1,334	1,410
Other	(2,626)	(3,644)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 65,492	$ 74,554
Percent of total fixed maturity and equity securities	0.50%	0.50%
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 530,806	$ 512,714
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	9,568	8,626
Acquisitions	15,800	8,000
Sales	0	0
Amortization	4	4
Other	(1,377)	(1,331)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 554,801	$ 528,013
Percent of total fixed maturity and equity securities	3.80%	3.60%
Equities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 870	$ 833
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	0	0
Acquisitions	0	0
Sales	0	0
Amortization	0	0
Other	0	0
Transfers in and/or out of Level 3	0	0
Ending balance	$ 870	$ 833
Percent of total fixed maturity and equity securities	0.00%	0.00%