Fair Value of Assets and Liabilities - Schedule of Assets and Liabilities Measure at Fair Value (Details) - Recurring - Marketable securities $ in Thousands	Sep. 30, 2018 USD ($)
Recurring Fair Value Measurements:
Assets, fair value	$ 248,838
Quoted Prices in Active Markets for Identical Assets (Level 1)
Recurring Fair Value Measurements:
Assets, fair value	248,838
Significant Other Observable Inputs (Level 2)
Recurring Fair Value Measurements:
Assets, fair value	0
Significant Unobservable Inputs (Level 3)
Recurring Fair Value Measurements:
Assets, fair value	$ 0