Fair Value Measurement - Financial Assets and Liabilities Accounted for at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, Level 1 to Level 2 Transfers, Amount	$ 0
Investments under executive deferred compensation plan	26,559,000	$ 25,494,000
Private equity securities	32,000	38,000
Private equity securities measured at net asset value	5,117,000	5,121,000
Foreign currency forward contracts, assets	247,000
Obligations under executive deferred compensation plan	26,559,000	25,494,000
Foreign currency forward contracts, liabilities		4,954,000
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	26,559,000	25,494,000
Private equity securities	32,000	38,000
Private equity securities measured at net asset value	0	0
Foreign currency forward contracts, assets	0
Obligations under executive deferred compensation plan	26,559,000	25,494,000
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	247,000
Obligations under executive deferred compensation plan	0	0
Foreign currency forward contracts, liabilities		4,954,000
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