Debt - Line of Credit (Details) - Revolving Credit Facility - Wells Fargo, Merrill Lynch, Pierce Fenner Smith, PNC Capital Markets Credit Facility - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2018
Line of Credit Facility [Line Items]
Long-term line of credit	$ 300,000,000
Issuance costs, net of accumulated amortization	(1,615,000)
Long-term Line of Credit, Net	298,385,000
5 Year - swapped to fixed rate \| Interest Rate Swap One
Line of Credit Facility [Line Items]
Long-term line of credit	$ 90,000,000
Stated interest rate (percent)	2.651%
Notional amount		$ 90,000,000
5 Year - swapped to fixed rate \| London Interbank Offered Rate (LIBOR) \| Interest Rate Swap One
Line of Credit Facility [Line Items]
Basis spread on variable rate, derivative	1.30%
5 year - swapped to fixed rate \| Interest Rate Swap Two
Line of Credit Facility [Line Items]
Long-term line of credit	$ 60,000,000
Stated interest rate (percent)	2.6525%
Notional amount		$ 60,000,000
5 year - swapped to fixed rate \| London Interbank Offered Rate (LIBOR) \| Interest Rate Swap Two
Line of Credit Facility [Line Items]
Basis spread on variable rate, derivative	1.30%
5 Year - variable rate
Line of Credit Facility [Line Items]
Long-term line of credit	$ 50,000,000
Stated interest rate (percent)	2.6607%
5 Year - variable rate \| London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate, debt instrument	1.30%
7 Year - variable rate
Line of Credit Facility [Line Items]
Long-term line of credit	$ 100,000,000
Stated interest rate (percent)	2.9607%
7 Year - variable rate \| London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Basis spread on variable rate, debt instrument	1.60%

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Long-term Line of Credit, Net

+ References

No definition available.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

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- Definition

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

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- Definition

Amount, after accumulated amortization, of debt issuance costs related to line of credit arrangements. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section S45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=68176171&loc=SL68176184-208336

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- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-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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-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

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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