Fair Value Measurements and Derivative Instruments (Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	$ 667,638	$ 919,371
Total liabilities	(1,402)	(735)
Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	401,805	612,647
Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	21,979	19,133
Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	20,060	21,711
Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	82,050	108,577
Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	72,149	56,938
Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	66,339	98,334
Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	3,256	2,031
Total liabilities	(1,402)	(735)
Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	227,669	321,116
Total liabilities	0	0
Level 1 | Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	161,441	220,756
Level 1 | Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 1 | Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	1,656	1,570
Level 1 | Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	64,572	98,790
Level 1 | Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 1 | Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 1 | Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Total liabilities	0	0
Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	439,969	598,255
Total liabilities	(1,402)	(735)
Level 2 | Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	240,364	391,891
Level 2 | Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	21,979	19,133
Level 2 | Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	18,404	20,141
Level 2 | Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	17,478	9,787
Level 2 | Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	72,149	56,938
Level 2 | Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	66,339	98,334
Level 2 | Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	3,256	2,031
Total liabilities	(1,402)	(735)
Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Total liabilities	0	0
Level 3 | Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 3 | Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 3 | Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 3 | Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 3 | Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 3 | Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Level 3 | Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets	0	0
Total liabilities	$ 0	$ 0