Fair Value Measurements - Recurring (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liabilities
Embedded derivative liability	$ 2,508	$ 3,134	$ 10,900
Recurring [Member]
Assets
Cash surrender value	8,057	8,057
Derivative contracts	8,493	4,218
Total	16,550	13,025
Liabilities
Derivative contracts	7,017	8,836
Embedded derivative liability	2,508	3,134
Total	9,525	11,970
Recurring [Member] \| Certificates of Deposit [Member]
Assets
Certificates of deposit		750
Recurring [Member] \| Level 2 [Member]
Assets
Cash surrender value	8,057	8,057
Derivative contracts	8,493	4,218
Total	16,550	13,025
Liabilities
Derivative contracts	7,017	8,836
Total	7,017	8,836
Recurring [Member] \| Level 2 [Member] \| Certificates of Deposit [Member]
Assets
Certificates of deposit		750
Recurring [Member] \| Level 3 [Member]
Liabilities
Embedded derivative liability	2,508	3,134
Total	$ 2,508	$ 3,134

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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The carrying amount of securities categorized neither as held-to-maturity nor trading which serve as security for borrowings or other arrangements.

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Fair value portion of the amount that could be realized under a life insurance contract or contracts owned by the entity, commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI).

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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