Revolving Credit Facilities and Mortgage Payable - Primary Credit Facility (Details) - Line of Credit	1 Months Ended		6 Months Ended
	Nov. 02, 2018 USD ($)	Sep. 30, 2018 USD ($) day	Sep. 30, 2018 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		3.51%	3.51%
Primary Credit Facility | Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Interest rate, effective percentage		5.50%	5.50%
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			10,000,000
Outstanding letters of credit		$ 549,000	549,000
Proceeds from lines of credit			62,000,000
Long-term line of credit		52,000,000	52,000,000
Amount available under the credit agreement		$ 347,451,000	347,451,000
Primary Credit Facility | Revolving Credit Facility | Subsequent Event
Notes Payable and Long-Term Debt
Repayments of lines of credit	$ 12,000,000
Outstanding letters of credit	549,000
Proceeds from lines of credit	54,000,000
Long-term line of credit	94,000,000
Amount available under the credit agreement	$ 305,451,000
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		$ 400,000	400,000
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		$ 1,300,000	$ 1,300,000