Fair Value Measurements - Assets Measured for at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 840,028	$ 820,438
Equity securities	53,175	63,277
Mortgage-Backed and Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	331,767	309,597
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	893,203	883,715
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]
Fair Value	65,642	66,637
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	427,433	428,865
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]
Fair Value	331,767	309,597
Fair Value, Measurements, Recurring [Member] \| Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,420	3,362
Fair Value, Measurements, Recurring [Member] \| Redeemable Preferred Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11,766	11,977
Fair Value, Measurements, Recurring [Member] \| Common Stock [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	4,342	15,564
Fair Value, Measurements, Recurring [Member] \| Mutual funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	48,833	47,713
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	53,175	63,277
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	4,342	15,564
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	48,833	47,713
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	840,028	820,438
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]
Fair Value	65,642	66,637
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	427,433	428,865
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]
Fair Value	331,767	309,597
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member] \| Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,420	3,362
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]
Fair Value	$ 11,766	$ 11,977

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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-Publisher FASB
-Name Accounting Standards Codification
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