Company's Invested Assets and Derivative Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015		Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	$ 1,449,056		$ 1,439,186
Common Shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	110,315		122,048
Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,306,149		1,283,475
Fixed Maturities | U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	107,122		80,767
Fixed Maturities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	205,240		191,473
Fixed Maturities | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	159,123		208,759
Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	140,390		133,158
Fixed Maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	260,022		178,263
Fixed Maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	332,111		383,416
Fixed Maturities | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	102,141		107,639
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,416,464	[1]	1,405,523
Total invested liabilities	15,256		13,675
Fair Value, Measurements, Recurring | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	15,256		13,675
Fair Value, Measurements, Recurring | Common Shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	110,315		122,048
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,306,149		1,283,475
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	107,122		80,767
Fair Value, Measurements, Recurring | Fixed Maturities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	205,240		191,473
Fair Value, Measurements, Recurring | Fixed Maturities | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	159,123		208,759
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	140,390		133,158
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	260,022		178,263
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	332,111		383,416
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	102,141		107,639
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	211,579	[1]	196,813
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Common Shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	110,315		122,048
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	101,264		74,765
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Fixed Maturities | U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	101,264		74,765
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,204,885	[1]	1,208,710
Total invested liabilities	15,256		13,675
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	15,256		13,675
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,204,885		1,208,710
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	5,858		6,002
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	205,240		191,473
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	159,123		208,759
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	140,390		133,158
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	260,022		178,263
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	332,111		383,416
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Fixed Maturities | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	$ 102,141		$ 107,639

[1] Excluded from the table above are limited partnerships of $32.6 million at December 31, 2015 whose fair value is based on net asset value as a practical expedient. Based on new accounting guidance adopted this quarter, these investments are excluded from the hierarchy table.