Fair Value Measurements - Assets Measured for at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 855,284	$ 820,438
Equity securities	43,717	63,277
Mortgage-backed and asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	311,079	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	899,001	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	54,364	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	476,218	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	311,079	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	3,496	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	10,127	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	2,377	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	41,340	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	43,717	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	2,377	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	41,340	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	855,284	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	54,364	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	476,218	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	311,079	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	3,496	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	$ 10,127	$ 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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