Revolving Credit Facilities and Mortgage Payable - Primary Credit Facility (Details) - Line of Credit	1 Months Ended		3 Months Ended		9 Months Ended
	Feb. 01, 2019 USD ($)	Sep. 30, 2018 USD ($) day	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 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			3.75%		3.75%
Primary Credit Facility | Alternate Base Rate based interest rates
Notes Payable and Long-Term Debt
Interest rate, effective percentage			5.75%		5.75%
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		$ 1,292,000		$ 1,292,000