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, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 576,779	$ 359,281
Total gains or losses:
Included in realized gains/losses	1,182	15,925
Included in other comprehensive income	9,702	23,953
Acquisitions	38,600	186,365
Sales		(16,050)
Amortization	4,197	4,082
Other	(17,819)	(5,004)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 612,641	$ 568,552
Percent of total fixed maturity and equity securities	4.30%	4.00%
Collateralized debt obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 63,232	$ 58,205
Total gains or losses:
Included in realized gains/losses	0	15,924
Included in other comprehensive income	12,797	3,929
Acquisitions	0	0
Sales		(16,049)
Amortization	4,183	4,072
Other	(8,100)	(3,231)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 72,112	$ 62,850
Percent of total fixed maturity and equity securities	0.50%	0.40%
Corporates [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 512,714	$ 300,300
Total gains or losses:
Included in realized gains/losses	1,182	1
Included in other comprehensive income	(3,121)	19,967
Acquisitions	38,600	186,365
Sales		(1)
Amortization	14	10
Other	(9,719)	(1,773)
Transfers in and/or out of Level 3	0	0
Ending balance	$ 539,670	$ 504,869
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	$ 833	$ 776
Total gains or losses:
Included in realized gains/losses	0	0
Included in other comprehensive income	26	57
Acquisitions	0	0
Sales		0
Amortization	0	0
Other	0	0
Transfers in and/or out of Level 3	0	0
Ending balance	$ 859	$ 833
Percent of total fixed maturity and equity securities	0.00%	0.00%