Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Parenthetical) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Money market funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Realized gains on the investments	$ 0
Unrealized gains on the investments	0
Other mutual funds
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains on the investments	1,000,000	$ 700,000
Unrealized gains on the investments after-tax	$ 600,000	$ 400,000