Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Investment [Line Items]
Beginning Balance	$ 576,779	$ 359,281
Included in realized gains/losses	0	15,925
Included in other comprehensive income	5,105	28,272
Acquisitions	19,400	90,680
Sales		(16,050)
Amortization	2,817	2,673
Other	(7,571)	(4,396)
Transfers in and/or out of Level 3	0	0
Ending Balance	$ 596,530	$ 476,385
Percent of total fixed maturity and equity securities	4.20%	3.40%
Collateralized Debt Obligations [Member]
Investment [Line Items]
Beginning Balance	$ 63,232	$ 58,205
Included in realized gains/losses	0	15,924
Included in other comprehensive income	13,531	8,702
Acquisitions	0	0
Sales		(16,049)
Amortization	2,810	2,668
Other	(5,905)	(3,231)
Transfers in and/or out of Level 3	0	0
Ending Balance	$ 73,668	$ 66,219
Percent of total fixed maturity and equity securities	0.50%	0.50%
Corporates [Member]
Investment [Line Items]
Beginning Balance	$ 512,714	$ 300,300
Included in realized gains/losses	0	1
Included in other comprehensive income	(8,426)	19,513
Acquisitions	19,400	90,680
Sales		(1)
Amortization	7	5
Other	(1,666)	(1,165)
Transfers in and/or out of Level 3	0	0
Ending Balance	$ 522,029	$ 409,333
Percent of total fixed maturity and equity securities	3.70%	2.90%
Equity Securities [Member]
Investment [Line Items]
Beginning Balance	$ 833	$ 776
Included in realized gains/losses	0	0
Included in other comprehensive income	0	57
Acquisitions	0	0
Sales		0
Amortization	0	0
Other	0	0
Transfers in and/or out of Level 3	0	0
Ending Balance	$ 833	$ 833
Percent of total fixed maturity and equity securities	0.00%	0.00%