Fair Value Measurements and Derivative Instruments (Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	$ 747,177	$ 748,738
Liabilities	(3,717)	(5,387)
Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	232,829	143,450
Liabilities	0	0
Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	514,348	605,288
Liabilities	(3,717)	(5,387)
Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Liabilities	0	0
Money market funds / commercial paper
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	331,646	343,994
Money market funds / commercial paper | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	114,471	41,215
Money market funds / commercial paper | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	217,175	302,779
Money market funds / commercial paper | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Equity securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	24,571	24,538
Equity securities | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Equity securities | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	24,571	24,538
Equity securities | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Commingled fixed income securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	23,668	22,571
Commingled fixed income securities | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	1,536	0
Commingled fixed income securities | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	22,132	22,571
Commingled fixed income securities | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Debt securities - U.S. and foreign governments, agencies and municipalities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	136,180	114,801
Debt securities - U.S. and foreign governments, agencies and municipalities | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	116,822	102,235
Debt securities - U.S. and foreign governments, agencies and municipalities | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	19,358	12,566
Debt securities - U.S. and foreign governments, agencies and municipalities | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Debt securities - corporate
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	69,891	62,884
Debt securities - corporate | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Debt securities - corporate | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	69,891	62,884
Debt securities - corporate | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Mortgage-backed / asset-backed securities
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	158,996	178,234
Mortgage-backed / asset-backed securities | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Mortgage-backed / asset-backed securities | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	158,996	178,234
Mortgage-backed / asset-backed securities | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Interest rate swaps
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	1,588
Interest rate swaps | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0
Interest rate swaps | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	1,588
Interest rate swaps | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0
Foreign exchange contracts
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	637	1,716
Liabilities	(3,717)	(5,387)
Foreign exchange contracts | Level 1
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Liabilities	0	0
Foreign exchange contracts | Level 2
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	637	1,716
Liabilities	(3,717)	(5,387)
Foreign exchange contracts | Level 3
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Assets	0	0
Liabilities	$ 0	$ 0