Summary of Consolidated Investments-Other Than Investments in Related Parties (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	$ 2,550,229	$ 2,191,827
Total investments and cash	2,820,486
Gross Unrealized Gains	83,078	69,554
Gross Unrealized Losses	(11,672)	(33,530)
Amortized Cost	2,749,080	2,538,562
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	179,506
Gross Unrealized Losses	(21)
Amortized Cost	179,527	296,250
Cash
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	90,751
Amortized Cost	90,751	86,509
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	2,365,934	2,047,873
Gross Unrealized Gains	52,322	43,854
Gross Unrealized Losses	(10,347)	(32,980)
Amortized Cost	2,323,959	2,036,999
Fixed maturities | U.S. Treasury bonds, agency bonds, and foreign
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	397,923	441,685
Gross Unrealized Gains	3,431	2,854
Gross Unrealized Losses	(5,965)	(8,855)
Amortized Cost	400,457	447,686
Fixed maturities | States, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	541,007	460,422
Gross Unrealized Gains	19,204	9,298
Gross Unrealized Losses	(558)	(13,651)
Amortized Cost	522,361	464,775
Fixed maturities | Agency mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	364,622	301,274
Gross Unrealized Gains	8,476	6,779
Gross Unrealized Losses	(998)	(6,016)
Amortized Cost	357,144	300,511
Fixed maturities | Residential Mortgage Backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	34,087	41,755
Gross Unrealized Gains	1,153	1,212
Gross Unrealized Losses	(138)	(161)
Amortized Cost	33,072	40,704
Fixed maturities | Asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	206,413	125,133
Gross Unrealized Gains	380	653
Gross Unrealized Losses	(964)	(480)
Amortized Cost	206,997	124,960
Fixed maturities | Commercial mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	206,318	172,750
Gross Unrealized Gains	6,630	7,656
Gross Unrealized Losses	(98)	(374)
Amortized Cost	199,786	165,468
Fixed maturities | Mortgage-backed and asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	811,440	640,912
Gross Unrealized Gains	16,639	16,300
Gross Unrealized Losses	(2,198)	(7,031)
Amortized Cost	796,999	631,643
Fixed maturities | Corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	615,564	504,854
Gross Unrealized Gains	13,048	15,402
Gross Unrealized Losses	(1,626)	(3,443)
Amortized Cost	604,142	492,895
Equity Securities | Common Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	127,183	143,954
Gross Unrealized Gains	28,520	25,700
Gross Unrealized Losses	(1,254)	(550)
Amortized Cost	99,917	118,804
Equity Securities | Preferred Stock
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Total investments and cash	57,112
Gross Unrealized Gains	2,236
Gross Unrealized Losses	(50)
Amortized Cost	$ 54,926