Fair Value Measurement - Financial Assets and Liabilities Accounted for at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments under executive deferred compensation plan	$ 27,871	$ 25,494
Private equity securities	31	38
Private equity securities measured at net asset value	7,194	5,121
Foreign currency forward contracts, assets	142
Obligations under executive deferred compensation plan	27,871	25,494
Foreign currency forward contracts, liabilities		4,954
Quoted Prices in Active Markets for Identical Items (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments under executive deferred compensation plan	27,871	25,494
Private equity securities	31	38
Private equity securities measured at net asset value	0	0
Foreign currency forward contracts, assets	0
Obligations under executive deferred compensation plan	27,871	25,494
Foreign currency forward contracts, liabilities		0
Quoted Prices in Active Markets for Similar Items (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments under executive deferred compensation plan	0	0
Private equity securities	0	0
Private equity securities measured at net asset value	0	0
Foreign currency forward contracts, assets	142
Obligations under executive deferred compensation plan	0	0
Foreign currency forward contracts, liabilities		4,954
Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments under executive deferred compensation plan	0	0
Private equity securities	0	0
Private equity securities measured at net asset value	0	0
Foreign currency forward contracts, assets	0
Obligations under executive deferred compensation plan	$ 0	0
Foreign currency forward contracts, liabilities		$ 0