Retirement and Other Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended				24 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Minimum allowed employee contributions, percent of salary	1.00%
Maximum allowed employee contributions, percent of salary	50.00%
Discretionary contributions, vesting period (in years)	5 years
Deferred compensation, annual benefit, percent of compensation, Benefit Restoration Plan		4.00%			3.00%
Defined contribution plans expense	$ 45.9		$ 51.0	$ 37.9
Deferred compensation, common stock authorized for issuance (shares)	2,960,000
Deferred compensation, common stock issued (shares)	0
Deferred compensation liability	$ 31.1		$ 28.0
Employee contribution, first 3% of salary
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of match		100.00%			100.00%
Employer matching contribution, percent of salary		3.00%			3.00%
Employee contribution, additional 2% of salary
Defined Benefit Plan Disclosure [Line Items]
Employer matching contribution, percent of match		50.00%
Employer matching contribution, percent of salary		2.00%