Fair Value Measurements and Derivative Instruments (Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	$ 1,102,556	$ 747,177
Liabilities	(335)	(3,717)
Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	685,917	331,646
Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	40,717	24,571
Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	6,085	23,668
Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	134,628	136,180
Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	75,109	69,891
Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	158,202	158,996
Interest rate swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	1,776	1,588
Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	122	637
Liabilities	(335)	(3,717)
Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	260,959	232,829
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]
Assets	143,349	114,471
Level 1 | Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Level 1 | Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	1,569	1,536
Level 1 | Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	116,041	116,822
Level 1 | Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
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]
Assets	0	0
Level 1 | Interest rate swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Level 1 | Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Liabilities	0	0
Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	841,597	514,348
Liabilities	(335)	(3,717)
Level 2 | Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	542,568	217,175
Level 2 | Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	40,717	24,571
Level 2 | Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	4,516	22,132
Level 2 | Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	18,587	19,358
Level 2 | Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	75,109	69,891
Level 2 | Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	158,202	158,996
Level 2 | Interest rate swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	1,776	1,588
Level 2 | Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	122	637
Liabilities	(335)	(3,717)
Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
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]
Assets	0	0
Level 3 | Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Level 3 | Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Level 3 | Government and related securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Level 3 | Corporate debt securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
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]
Assets	0	0
Level 3 | Interest rate swap
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Level 3 | Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Liabilities	$ 0	$ 0