Company's Invested Assets and Derivative Instruments Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	$ 1,426,709		$ 1,449,056
Common Shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	120,557		110,315
Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,240,031		1,306,149
Fixed Maturities | U.S. Treasury and Agency Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	72,047		107,122
Fixed Maturities | Obligations of States and Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	156,446		205,240
Fixed Maturities | Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	88,468		159,123
Fixed Maturities | Commercial Mortgage-Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	183,192		140,390
Fixed Maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	233,991		260,022
Fixed Maturities | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	380,027		332,111
Fixed Maturities | Foreign Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	125,860		102,141
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,360,588	[1]	1,416,464	[2]
Total invested liabilities	11,524		15,256
Fair Value, Measurements, Recurring | Derivative instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested liabilities	11,524		15,256
Fair Value, Measurements, Recurring | Common Shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	120,557		110,315
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	1,240,031		1,306,149
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	72,047		107,122
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	156,446		205,240
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	88,468		159,123
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	183,192		140,390
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	233,991		260,022
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total invested assets	380,027		332,111
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	125,860		102,141
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	192,604	[1]	211,579	[2]
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	120,557		110,315
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	72,047		101,264
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	72,047		101,264
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	1,167,984	[1]	1,204,885	[2]
Total invested liabilities	11,524		15,256
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	11,524		15,256
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	1,167,984		1,204,885
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
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	156,446		205,240
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	88,468		159,123
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	183,192		140,390
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	233,991		260,022
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	380,027		332,111
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	$ 125,860		$ 102,141

[1] Excluded from the table above are limited partnerships of $66.1 million at December 31, 2016 whose fair value is based on net asset value as a practical expedient.

[2] 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.