Segment Information - Additional Information (Detail) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Segment	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Acquisition And Operating Expenses	$ 109.9	$ 107.2	$ 222.1	$ 214.7
Number of reportable segments (segments) \| Segment			4
Investable assets	$ 2,733.5		$ 2,733.5		$ 2,663.7

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