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Senior Bank Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument
Term loan outstanding	$ 779,700,000	$ 809,000,000
Unsecured Debt | Revolver
Debt Instrument
Maximum borrowing capacity	1,450,000,000.00
Maximum borrowing capacity, additional amount	350,000,000
Outstanding borrowings	$ 330,000,000	$ 425,000,000
Weighted average interest rate	3.92%	2.76%
Commitment fee amount	$ 1,600,000	$ 1,500,000
Unsecured Debt | Term Loan
Debt Instrument
Maximum borrowing capacity	450,000,000.0
Maximum borrowing capacity, additional amount	75,000,000.0
Term loan outstanding	$ 450,000,000	$ 385,000,000
Weighted average interest rate on amount outstanding	3.33%	2.69%

X

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Additional Amount

+ References

No definition available.

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X

- 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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- Definition

Weighted average interest rate of debt outstanding.

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X

- 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))
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X

- Definition

Amount of the fee for available but unused credit capacity under the credit facility.

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-Name Accounting Standards Codification
-Topic 210
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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

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