Investments - Schedule of Changes in Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Investment [Line Items]
Beginning Balance	$ 576,779	$ 359,281
Included in realized gains/losses	0	15,925
Included in other comprehensive income	22,182	20,055
Acquisitions	8,000	33,791
Sales	0	(16,050)
Amortization	1,414	1,310
Other	(4,975)	(3,176)
Transfers in and/or out of Level 3	0	0
Ending Balance	603,400	411,136
Percent of total fixed maturity and equity securities	4.10%	3.00%
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	13,556	7,283
Acquisitions	0	0
Sales	0	(16,049)
Amortization	1,410	1,307
Other	(3,644)	(2,340)
Transfers in and/or out of Level 3	0	0
Ending Balance	74,554	64,330
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	8,626	12,772
Acquisitions	8,000	33,791
Sales	0	(1)
Amortization	4	3
Other	(1,331)	(836)
Transfers in and/or out of Level 3	0	0
Ending Balance	528,013	346,030
Percent of total fixed maturity and equity securities	3.60%	2.50%
Equity Securities [Member]
Investment [Line Items]
Beginning Balance	833	776
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	$ 833	$ 776
Percent of total fixed maturity and equity securities	0.00%	0.00%