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Fair Value Measurements - Assets Measured for at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities $ 634,238 $ 639,334
Equity securities 66,043 62,215
Available-for-sale short-term investments 10,000 10,000
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets accounted for at fair value 710,281 711,549
Fair Value, Measurements, Recurring [Member] | U.S. Government Obligations and Agencies [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 58,797 59,604
Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 298,730 227,504
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed and Asset-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 229,499 219,452
Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 34,284 120,295
Fair Value, Measurements, Recurring [Member] | Redeemable Preferred Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 12,928 12,479
Fair Value, Measurements, Recurring [Member] | Common Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 16,578 18,811
Fair Value, Measurements, Recurring [Member] | Mutual Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 49,465 43,404
Fair Value, Measurements, Recurring [Member] | Available For Sale [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale short-term investments 10,000 10,000
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets accounted for at fair value 66,043 62,215
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Common Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 16,578 18,811
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Mutual Funds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 49,465 43,404
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total assets accounted for at fair value 644,238 649,334
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Government Obligations and Agencies [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 58,797 59,604
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 298,730 227,504
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-Backed and Asset-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 229,499 219,452
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 34,284 120,295
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Redeemable Preferred Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 12,928 12,479
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Available For Sale [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale short-term investments $ 10,000 $ 10,000