Segment Information - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Acquisition And Operating Expenses	$ 123.0	$ 109.7	$ 357.4	$ 325.2
Investable assets	$ 2,782.2		$ 2,782.2		$ 2,747.3