Related Parties - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Notes Receivable Related Parties [Line Items]
Lease paid by company	$ 38.4	$ 37.0	$ 35.7
Fees earned under administrative service agreement	156.4	144.8	137.5
Director [Member]
Notes Receivable Related Parties [Line Items]
Lease paid by company	3.3	3.2	2.7
Akin Gump [Member]
Notes Receivable Related Parties [Line Items]
Legal charges paid	$ 0.2	$ 0.1	$ 0.2