Fair Value of Financial Instruments - Additional Information (Detail) $ in Millions	1 Months Ended		3 Months Ended
	May 31, 2019 Agreement	May 31, 2009 Agreement	Mar. 31, 2019	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Fair Value Measurements Disclosure [Line Items]
Other invested assets				$ 54.6	$ 69.7
Other invested assets, at cost				12.5	6.5
Number of Credit Agreements | Agreement	2	2
Notes receivable from affiliate				70.0	70.0
Level 3 [Member]
Fair Value Measurements Disclosure [Line Items]
Federal home loan bank stock				12.0
Other invested assets, at cost				$ 0.5
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities				1
Other invested assets				$ 42.6	$ 56.4
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate				$ 70.0
Affiliated Entity [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable interest rate (percent)			7.00%