Debt - Schedule of Maturity Amounts Outstanding Under Credit Agreement , Third Restated Prudential Note Purchase Agreement (Detail) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Borrowings under credit agreement, outstanding amount	$ 86,863	$ 154,502
Borrowings under note purchase and guarantee agreement, outstanding amount	324,351	224,656
Total	415,000	380,000
Unamortized debt issuance costs, net	(3,786)	(842)
Total debt, net	$ 411,214	$ 379,158
Revolving Facility [Member]
Line of Credit Facility [Line Items]
Maturity Date	Mar. 23, 2022	Mar. 23, 2022
Interest Rate	3.53%	3.53%
Borrowings under credit agreement, outstanding amount	$ 40,000	$ 105,000
Total	$ 40,000
Term Loan [Member]
Line of Credit Facility [Line Items]
Maturity Date	Mar. 23, 2023	Mar. 23, 2023
Interest Rate	3.54%	3.54%
Borrowings under credit agreement, outstanding amount	$ 50,000	$ 50,000
Total	$ 50,000
Series A Notes Maturing in February 2021 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Feb. 25, 2021	Feb. 25, 2021
Interest Rate	6.00%	6.00%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 100,000	$ 100,000
Series B Notes Maturing in June 2023 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 02, 2023	Jun. 02, 2023
Interest Rate	5.35%	5.35%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 75,000	$ 75,000
Series C Notes Maturing in February 2025 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Feb. 25, 2025	Feb. 25, 2025
Interest Rate	4.75%	4.75%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000	$ 50,000
Series D Notes Maturing in June 2028 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 21, 2028	Jun. 21, 2028
Interest Rate	5.47%	5.47%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000
Series E Notes Maturing in June 2028 [Member]
Line of Credit Facility [Line Items]
Maturity Date	Jun. 21, 2028	Jun. 21, 2028
Interest Rate	5.47%	5.47%
Borrowings under note purchase and guarantee agreement, outstanding amount	$ 50,000

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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- Definition

Amount of unamortized debt discount (premium) and debt issuance costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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