Marketable Securities - Maturity Period (Details) $ in Thousands	2 Months Ended
	Sep. 30, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Due within one year	$ 38,139
Variable rate demand notes
Debt Securities, Available-for-sale [Line Items]
Due within one year	$ 8,700
Variable rate demand notes | Minimum
Debt Securities, Available-for-sale [Line Items]
Contractual maturities (in years)	19 years
Settlement basis period (in days)	1 day
Effective maturity period (in days)	1 day
Variable rate demand notes | Maximum
Debt Securities, Available-for-sale [Line Items]
Contractual maturities (in years)	31 years
Settlement basis period (in days)	7 days
Effective maturity period (in days)	7 days