Commitments, Contingencies and Guarantees (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Payment Required Upon Termination Of Employment [Line Items]
Minimum purchase commitments	$ 4,000,000.0
Damages sought	50,000,000
Loss contingency accrual	27,500,000
Outstanding letters of credit to insurance company as security for workers' compensation claims	$ 1,400,000
Period for continued vesting of outstanding equity awards upon termination of CEO without cause other than following a change in control, years	two years
Share-based compensation accelerated expense	$ 5,600,000
Liabilities for indemnification obligations	0	$ 0
Payment required to CEO upon termination of employment without cause other than following change in control
Payment Required Upon Termination Of Employment [Line Items]
Employee agreement contingencies	1,700,000
Payment required to officers upon termination of employment following change of control
Payment Required Upon Termination Of Employment [Line Items]
Employee agreement contingencies	37,700,000
Employee healthcare insurance policy
Payment Required Upon Termination Of Employment [Line Items]
General insurance expense	59,500,000	59,300,000	$ 52,700,000
Estimated liability for healthcare claims	5,500,000	5,000,000.0
Maximum | Employee healthcare insurance policy
Payment Required Upon Termination Of Employment [Line Items]
Retained claim liability per incident	$ 1,000,000	$ 1,000,000	$ 1,000,000