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Schedule of Amortized Cost and Estimated Fair Value of Investments (Detail) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		$ 1,498,024	$ 1,445,879
Gross Unrealized Gains		1,692	23,021
Gross Unrealized Losses		(20,650)	(9,414)
Estimated Fair Value		1,479,066	1,459,486
Other than temporary impairments recognized in AOCI	[1]	(1)	(1)
Common Shares
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		133,911	124,915
Gross Unrealized Gains			18,574
Gross Unrealized Losses			(3,260)
Estimated Fair Value		133,911	140,229
Other Invested Assets
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		82,159	77,820
Estimated Fair Value		82,159	77,820
Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		1,281,954	1,243,144
Gross Unrealized Gains		1,692	4,447
Gross Unrealized Losses		(20,650)	(6,154)
Estimated Fair Value		1,262,996	1,241,437
Other than temporary impairments recognized in AOCI	[1]	(1)	(1)
Fixed Maturities | U.S. Treasury and Agency Obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		99,713	105,311
Gross Unrealized Gains		476	562
Gross Unrealized Losses		(2,060)	(1,193)
Estimated Fair Value		98,129	104,680
Fixed Maturities | Obligations of States and Political Subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		95,708	94,947
Gross Unrealized Gains		304	441
Gross Unrealized Losses		(601)	(274)
Estimated Fair Value		95,411	95,114
Fixed Maturities | Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		176,983	150,237
Gross Unrealized Gains		313	404
Gross Unrealized Losses		(3,511)	(1,291)
Estimated Fair Value		173,785	149,350
Fixed Maturities | Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		203,834	203,827
Gross Unrealized Gains		115	267
Gross Unrealized Losses		(1,204)	(393)
Estimated Fair Value		202,745	203,701
Other than temporary impairments recognized in AOCI	[1]	(1)	(1)
Fixed Maturities | Commercial Mortgage-Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		151,337	140,761
Gross Unrealized Gains		58	101
Gross Unrealized Losses		(3,554)	(1,067)
Estimated Fair Value		147,841	139,795
Fixed Maturities | Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		431,814	422,486
Gross Unrealized Gains		417	2,295
Gross Unrealized Losses		(7,424)	(1,391)
Estimated Fair Value		424,807	423,390
Fixed Maturities | Foreign Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		122,565	125,575
Gross Unrealized Gains		9	377
Gross Unrealized Losses		(2,296)	(545)
Estimated Fair Value		$ 120,278	$ 125,407
[1] Represents the total amount of other than temporary impairment losses relating to factors other than credit losses recognized in accumulated other comprehensive income ("AOCI").

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