Revolving Credit Facilities and Mortgage Payable - Primary Credit Facility (Details) - Line of Credit	1 Months Ended	2 Months Ended	3 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) day	May 17, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)
Primary Credit Facility
Notes Payable and Long-Term Debt
Additional borrowing capacity	$ 200,000,000			$ 200,000,000
Maximum borrowing capacity	$ 600,000,000			600,000,000
Primary Credit Facility | LIBOR based interest rates
Notes Payable and Long-Term Debt
Interest rate, effective percentage			3.62%		3.62%
Primary Credit Facility | Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Interest rate, effective percentage			5.63%		5.63%
Primary Credit Facility | Revolving Credit Facility
Notes Payable and Long-Term Debt
Term of agreement (in years)	5 years
Current borrowing capacity	$ 400,000,000			400,000,000
Number of days to deliver compliance certificate | day	45
Fee on unused amount of revolving credit facility (as a percentage)	0.15%
Repayments of lines of credit					$ 116,000,000
Outstanding letters of credit			$ 549,000		549,000
Proceeds from lines of credit					116,000,000
Long-term line of credit			0		0
Amount available under the credit agreement			399,451,000		399,451,000
Primary Credit Facility | Revolving Credit Facility | Subsequent Event
Notes Payable and Long-Term Debt
Outstanding letters of credit		$ 549,000
Proceeds from lines of credit		0
Long-term line of credit		0
Amount available under the credit agreement		$ 399,451,000
Primary Credit Facility | Revolving Credit Facility | LIBOR based interest rates
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	1.25%
Primary Credit Facility | Revolving Credit Facility | Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	0.25%
Primary Credit Facility | Revolving Credit Facility | Federal Funds Effective Swap Rate
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	0.50%
Primary Credit Facility | Revolving Credit Facility | One Month Adjusted LIBOR Rate
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	1.00%
Prior Credit Facility | Revolving Credit Facility
Notes Payable and Long-Term Debt
Repayments of lines of credit				27,000,000
Outstanding letters of credit			$ 549,000		$ 549,000
Deferred financing costs	$ 447,000			447,000
Minimum | Primary Credit Facility | Revolving Credit Facility
Notes Payable and Long-Term Debt
Fee on unused amount of revolving credit facility (as a percentage)			0.125%
Minimum | Primary Credit Facility | Revolving Credit Facility | LIBOR based interest rates
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	1.125%
Minimum | Primary Credit Facility | Revolving Credit Facility | Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	0.125%
Maximum | Primary Credit Facility
Notes Payable and Long-Term Debt
Capacity available for letters of credit	$ 25,000,000			25,000,000
Sublimit available for borrowings in foreign currency	$ 175,000,000			175,000,000
Maximum | Primary Credit Facility | Revolving Credit Facility
Notes Payable and Long-Term Debt
Fee on unused amount of revolving credit facility (as a percentage)			0.20%
Maximum | Primary Credit Facility | Revolving Credit Facility | LIBOR based interest rates
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	1.625%
Maximum | Primary Credit Facility | Revolving Credit Facility | Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Spread on variable interest rate (as a percent)	0.625%
Prepaid Expenses and Other Current Assets | Primary Credit Facility | Revolving Credit Facility
Notes Payable and Long-Term Debt
Deferred financing costs	$ 1,297,000			$ 1,297,000