Financing Activity - Mortgage Loan Activity Additional Detail (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Mortgage Loan Activity [Line Items]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.21
Maximum Unsecured Borrowing Amount Under Unencumbered Debt Yield Covenant	$ 189.7
Carrying Value Of Net Losses Associated With Interest Rate Swaps	0.4
Derivative, Notional Amount	748.6
Two Thousand Thirteen Revolving Credit Facility [Member]
Mortgage Loan Activity [Line Items]
Letters of Credit Outstanding, Amount	15.8
Secured Debt [Member]
Mortgage Loan Activity [Line Items]
Debt Issuance Costs, Line of Credit Arrangements, Net	$ 3.7	$ 3.4

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

Carrying Value Of Net Losses Associated With Interest Rate Swaps

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- Definition

Maximum Unsecured Borrowing Amount Under Unencumbered Debt Yield Covenant

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- Definition

Mortgage Loan Activity [Line Items]

+ References

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- Definition

Aggregate dividends paid during the period for each share of common stock outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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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://www.xbrl.org/2003/role/presentationRef
-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 total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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