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Fair Value Measurements - Assets Measured for at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities $ 760,408 $ 639,334
Equity securities 69,108 62,215
Available-for-sale short-term investments   10,000
U.S. Government Obligations and Agencies [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 65,998 59,604
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 409,967 227,504
Mortgage-Backed and Asset-Backed Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 269,228 219,452
Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 3,278 120,295
Redeemable Preferred Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 11,937 12,479
Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 829,516 711,549
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 65,998 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 409,967 227,504
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 269,228 219,452
Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 3,278 120,295
Fair Value, Measurements, Recurring [Member] | Redeemable Preferred Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities 11,937 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 20,575 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 48,533 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
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 69,108 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 20,575 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 48,533 43,404
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Equity securities 760,408 649,334
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 65,998 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 409,967 227,504
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 269,228 219,452
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 3,278 120,295
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 $ 11,937 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