Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Parenthetical) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains on the investments		$ 1,000,000
Unrealized gains on the investments after-tax		600,000
Unrealized Gains on Equity Securities Classified as Available-for-sale
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cumulative-effect adjustment recognized to beginning retained earnings	$ (616,000)
Unrealized Gains on Equity Securities Classified as Available-for-sale | Adoption of ASU 2016-01
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Cumulative-effect adjustment recognized to beginning retained earnings	600,000
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
Unrealized gains on the investments after-tax		$ 600,000