Income Taxes - Schedule of deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Deferred compensation	$ 458.2	$ 391.0
Fixed assets and intangible assets	107.8	0.0
Deferred revenue	20.2	102.4
Accrued expenses	53.3	38.3
Other	33.3	26.5
Deferred tax assets, gross	672.8	558.2
Valuation allowance	0.0	(4.2)
Total deferred tax assets	672.8	554.0
Deferred tax liabilities:
Fixed assets and intangible assets	0.0	(44.6)
Other	(2.7)	(3.1)
Total deferred tax liabilities	(2.7)	(47.7)
Net deferred tax assets	$ 670.1	$ 506.3