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Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Parenthetical) (Details) - USD ($)
6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Cash and Cash Equivalents [Line Items]    
Unrealized gains on the investments after-tax $ 516,000 $ 421,000
Money market funds    
Cash and Cash Equivalents [Line Items]    
Realized gains on the investments 0  
Unrealized gains on the investments 0  
Other mutual funds    
Cash and Cash Equivalents [Line Items]    
Unrealized gains on the investments 800,000 700,000
Unrealized gains on the investments after-tax $ 500,000 $ 400,000