Segment Information - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Acquisition And Operating Expenses	$ 110.9	$ 112.7	$ 333.0	$ 334.1
Number of reportable segments (segments) \| Segment			4
Investable assets	$ 2,697.5		$ 2,697.5		$ 2,781.2

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

No definition available.

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration