Fair Value Measurements - Summary of Fair Value Assets Measured on Recurring Basis (Parenthetical) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Unrealized gains on the investments after-tax	$ 564,000	$ 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	900,000	700,000
Unrealized gains on the investments after-tax	$ 600,000	$ 400,000