v3.8.0.1

Unsecured Credit Agreements - Summary of Unsecured Credit Agreements (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jun. 23, 2017	Dec. 31, 2017	Apr. 18, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying amount of debt		$ 1,181,470		$ 869,524
Long-term Debt		$ 1,181,470
2015 Unsecured Term Loan Agreement
Debt Instrument [Line Items]
Maturity Date	Feb. 28, 2019
2015 Unsecured Term Loan Agreement | Minimum | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	0.90%
2015 Unsecured Term Loan Agreement | Maximum | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
2017 Unsecured Revolving Credit and Term Loan Agreement | 5.5 Year Term Loan
Debt Instrument [Line Items]
Maturity Date	Jan. 31, 2023
2017 Unsecured Revolving Credit and Term Loan Agreement | 7 Year Term Loan
Debt Instrument [Line Items]
Maturity Date	Jun. 30, 2024
2017 Senior Notes
Debt Instrument [Line Items]
Debt instrument, interest rate			4.84%
Maturity Date		Apr. 18, 2017
Unsecured Debt
Debt Instrument [Line Items]
Carrying amount of debt		$ 1,113,000		762,000
Debt issuance costs, net		(3,088)		(2,109)
Long-term Debt		1,109,912		759,891
Unsecured Debt | 1 Month LIBOR
Debt Instrument [Line Items]
Carrying amount of debt				223,000
Unsecured Debt | 3 Month LIBOR
Debt Instrument [Line Items]
Carrying amount of debt				50,000
Unsecured Debt | 2015 Unsecured Term Loan Agreement
Debt Instrument [Line Items]
Carrying amount of debt		$ 325,000		375,000
Interest Rate		1-month LIBOR + 1.40%
Maturity Date		Feb. 28, 2019
Unsecured Debt | 2015 Unsecured Term Loan Agreement | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.40%
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement
Debt Instrument [Line Items]
Carrying amount of debt		$ 638,000
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | Revolver
Debt Instrument [Line Items]
Carrying amount of debt		$ 273,000
Interest Rate		1 and 3 month LIBOR + 1.20%
Maturity Date		Jan. 31, 2022
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | Revolver | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.20%
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | Revolver | 3 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.20%
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | 5.5 Year Term Loan
Debt Instrument [Line Items]
Carrying amount of debt		$ 265,000
Interest Rate		1-month LIBOR + 1.35%
Maturity Date		Jan. 31, 2023
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | 5.5 Year Term Loan | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.35%
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | 7 Year Term Loan
Debt Instrument [Line Items]
Carrying amount of debt		$ 100,000
Interest Rate		1-month LIBOR + 1.90%
Maturity Date		Jun. 30, 2024
Unsecured Debt | 2017 Unsecured Revolving Credit and Term Loan Agreement | 7 Year Term Loan | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.90%
Unsecured Debt | 2017 Senior Notes
Debt Instrument [Line Items]
Carrying amount of debt		$ 150,000
Debt instrument, interest rate		4.84%
Maturity Date		Apr. 30, 2027
Unsecured Debt | 2012 Unsecured Credit Agreement
Debt Instrument [Line Items]
Carrying amount of debt				202,000
Unsecured Debt | 2012 Unsecured Credit Agreement | Revolver
Debt Instrument [Line Items]
Carrying amount of debt				102,000
Interest Rate		1-month LIBOR + 1.45%
Maturity Date		Jun. 30, 2017
Unsecured Debt | 2012 Unsecured Credit Agreement | Revolver | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.45%
Unsecured Debt | 2012 Unsecured Credit Agreement | Term Note One
Debt Instrument [Line Items]
Carrying amount of debt				50,000
Interest Rate		3-month LIBOR + 1.45%
Maturity Date		Jun. 30, 2017
Unsecured Debt | 2012 Unsecured Credit Agreement | Term Note One | 3 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.45%
Unsecured Debt | 2012 Unsecured Credit Agreement | Term Note Two
Debt Instrument [Line Items]
Carrying amount of debt				50,000
Interest Rate		1-month LIBOR + 1.45%
Maturity Date		Jun. 30, 2017
Unsecured Debt | 2012 Unsecured Credit Agreement | Term Note Two | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.45%
Unsecured Debt | 2013 Unsecured Credit Agreement
Debt Instrument [Line Items]
Carrying amount of debt				$ 185,000
Interest Rate		1 month LIBOR + 1.75%
Maturity Date		Oct. 31, 2018
Unsecured Debt | 2013 Unsecured Credit Agreement | Minimum | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		1.75%
Unsecured Debt | 2013 Unsecured Credit Agreement | Maximum | 1 Month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate		2.50%

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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.

+ References

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

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+ References

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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.

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