Subsequent Events Debt issuance (Details) $ in Thousands	Feb. 05, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt instrument [Line Items]
Number of principal payments		5
Anniversary when principal payments begin		6
3.99% Senior unsecured notes [Member]
Debt instrument [Line Items]
Senior unsecured notes, interest rate		3.99%
Unsecured debt additional shelf note capacity		$ 50,000
Subsequent event [Member]
Debt instrument [Line Items]
Number of principal payments	5
Subsequent event [Member] | 4.00% Senior unsecured notes [Member]
Debt instrument [Line Items]
Senior unsecured notes, interest rate	4.00%
Unsecured debt additional shelf note capacity	$ 25,000
Subsequent event [Member] | 3.99% Senior unsecured notes [Member]
Debt instrument [Line Items]
Unsecured debt additional shelf note capacity allowed by credit facility	$ 25,000